Ch 5 Exploring Mixtures and their Separation Extra Questions Class 9 Topic-wise

“Exploring Mixtures and their Separation Extra Question”, has the following types of questions topic-wise:

• 1 mark questions
• 2 mark questions
• 3 mark questions
• 5 mark questions
• True False
• Fill in the blanks
• MCQs
• Assertion and Reason types question

In this “Exploring Mixtures and their Separation Extra Question” , we have created all the important questions from Chapter 5, Exploration, NCERT Class 9.

Before practising the question, ensure that you have gone through our short notes on the same chapter.

Also included the source of all the answers in the NCERT textbook chapter 5, Exploring Mixtures and their Separation Extra Question.

There may be some cross-topical questions. Two or more topics are mixed to form it.

5.1 How Can We Classify Mixtures?

1-Mark Questions

Question 1: What is a homogeneous mixture?

Answer: A homogeneous mixture has a uniform composition throughout. Every part of the mixture is identical.

Explanation: Just like sugar dissolved in water tastes equally sweet in the first and last sip, the composition never changes from one part to another.

Question 2: Give one example of a heterogeneous mixture.

Answer: Sand and water are an example of a heterogeneous mixture.

Explanation: Sand particles are visible in water and settle over time — the composition is not uniform throughout.

Question 3: What is another name for a homogeneous mixture?

Answer: A homogeneous mixture is also called a solution.

Explanation: Whether it is sugar water or vinegar, all solutions maintain uniform composition and are therefore homogeneous mixtures.

Question 4: Name the phenomenon observed when a laser beam is passed through a colloidal mixture.

Answer: The Tyndall effect is observed — the path of the laser beam becomes visible.

Explanation: Colloidal particles scatter light, making the beam’s path visible when viewed from the side perpendicular to the beam.

Question 5: Is a mixture of oil and water homogeneous or heterogeneous?

Answer: Oil and water form a heterogeneous mixture because they do not mix uniformly.

Explanation: Oil and water are immiscible — they form separate layers, making the mixture non-uniform throughout.

2-Mark Questions

Question 1 : Differentiate between a homogeneous and a heterogeneous mixture with one example each.

Answer:

Explanation: In a homogeneous mixture, you cannot distinguish the individual components. In a heterogeneous mixture, the components remain visibly distinct.

Question 2: In Activity 5.1, three mixtures were prepared — salt and water, chalk powder and water, and milk and water. Which of these would leave a residue on filter paper? Why?

Answer: Chalk powder and water would leave a residue on filter paper.
Chalk powder does not dissolve in water — it remains as suspended particles that are large enough to be trapped by filter paper.

Explanation: Salt dissolves completely (no residue). Milk particles are too tiny to be filtered. Only chalk particles are large enough to be captured.

Question 3: Why does a laser beam become visible when passed through milk mixed with water but not through salt water?

Answer: Milk is a colloid — its particles scatter light, making the laser beam’s path visible. This is called the Tyndall effect
Salt water is a true solution — its particles are too small to scatter light, so the beam is invisible.

Explanation: The Tyndall effect occurs only in colloids and suspensions, not in true solutions.

Question 4: Give two examples each of homogeneous and heterogeneous mixtures.

Answer:
Homogeneous Mixtures:

• Vinegar (acetic acid in water)
• Aerated drinks like soda (carbon dioxide in water)

Heterogeneous Mixtures:

• Sand and water
• Oil and water

Explanation: Homogeneous mixtures always have the same composition in every part. Heterogeneous mixtures have visibly different components.

Question 5: What prediction would you make if the three mixtures from Activity 5.1
(salt + water, chalk + water, milk + water) are left undisturbed for a few minutes?

Answer:

• Salt + water — No change. Salt remains dissolved.
• Chalk + water — Chalk particles settle at the bottom (sedimentation).
• Milk + water — No settling occurs. Milk particles stay evenly dispersed.

Explanation: Chalk forms a suspension (particles settle). Salt forms a solution (no settling). Milk forms a colloid (particles do not settle).

3-Mark Questions

Question 1: In Activity 5.1, three groups prepared different mixtures. Based on the laser light observations and filtration results, how would you classify the three mixtures? Justify your answer.

Answer:

Justification:

• Salt water — particles are too small to scatter light or be filtered → true solution
• Chalk water — large particles scatter light AND are filtered out → suspension
• Milk water — medium-sized particles scatter light but pass through filter paper → colloid

Explanation: These three results prove that all mixtures are not the same type — they differ in particle size, light scattering, and filterability.

Question 2: What is the Tyndall effect? Which of the mixtures in Activity 5.1 would show it, and why?

Answer: The Tyndall effect is the scattering of light by particles in a mixture, making the path of a light beam visible.

Mixtures that show the Tyndall effect:

• Chalk + water (suspension) ✅
• Milk + water (colloid) ✅

Mixture that does NOT show it:

• Salt + water (solution) ❌

Why?

• In colloids and suspensions, particles are larger and scatter the light beam.
• In a solution, particles are smaller than 1 nm — too tiny to scatter light.

Explanation: This is why you see bright rays of sunlight through leaves of a dense tree — dust and smoke particles in air scatter sunlight, showing the Tyndall effect.

Question 3: Why does a solution always remain homogeneous? Give two examples to support your answer.

Answer: A solution always remains homogeneous because the solute particles dissolve completely into the solvent at the molecular level. The particles are smaller than 1 nm in diameter — far too small to separate out or settle

Examples:

1. Sugar solution — equally sweet in every sip, from first to last
2. Vinegar — acetic acid is uniformly distributed in water at all points

Key reason: Since particles are uniformly distributed and cannot be seen or filtered, the composition stays the same throughout — making it permanently homogeneous.

Explanation: Unlike suspensions that settle or colloids that can scatter light, solutions remain stable and transparent indefinitely.

Question 4: How are suspensions different from colloids? Compare them on three properties.

Answer:

Brain Anchor: 🔥 Suspension settles. Colloid stays. Solution disappears into the liquid.

Explanation: The key difference is particle size and stability. Colloid particles are small enough to stay dispersed but large enough to scatter light.

Question 5: A student stirred sand into water and called it a solution. Is the student correct? Explain with three reasons.

Answer: ❌ The student is incorrect. Sand and water is a heterogeneous mixture (suspension), not a solution.

Three Reasons:

1. Visibility — Sand particles are visible to the naked eye; solute in a solution is not visible.
2. Settling — Sand settles at the bottom when left undisturbed; a true solution never settles.
3. Filtration — Sand can be separated by filtration; a true solution cannot.

Explanation: A solution requires the solute to completely dissolve in the solvent, forming a uniform mixture. Sand does not dissolve in water.

5-Mark Questions

Question 1: Describe Activity 5.1 in detail. What were the observations for each group, and what conclusions were drawn about the types of mixtures?

Answer:

🔬 Activity 5.1 — Classifying Mixtures

Aim: To classify three different mixtures based on their properties.

Preparation

Observations

Conclusions

Group A — Salt + Water = Solution

• Particles completely dissolve → homogeneous mixture
• No Tyndall effect — particles too small
• No residue on filter paper

Group B — Chalk + Water = Suspension

• Particles are visible and settle on a standing → heterogeneous mixture
• Shows the Tyndall effect
• Leaves residue on filter paper

Group C — Milk + Water = Colloid

• Particles are not visible, but do not settle
• Shows Tyndall effect (scattered light visible)
• No residue on filter paper — particles too small to be filtered

Final Conclusion

These are three different types of mixtures — not the same — classified by particle size, light scattering, and filterability.

Brain Anchor: 🔥 Salt hides. Chalk settles. Milk glows. Three mixtures. Three stories.

Question 2: Compare solutions, suspensions, and colloids based on their nature, particle size, visibility, Tyndall effect, settling, and filtration. Give two examples of each.

Answer:

🧪 Comparing the Three Types of Mixtures

Quick Summary Table

Why Does This Matter?

Brain Anchor: 🔥 Size decides everything — tiny hides, medium glows, large settles.

MCQs (Multiple Choice Questions)

Question 1: Which of the following is a homogeneous mixture?

(a) Sand and water
(b) Chalk and water
(c) Sugar and water
(d) Oil and water

Answer: (c) Sugar and water

Explanation: Sugar completely dissolves in water, forming a uniform solution (homogeneous mixture). The other options all form heterogeneous mixtures where components are visibly or physically distinct

Question 2: A laser beam is passed through three mixtures — salt water (A), chalk water (B), and milk water (C). In which mixtures will the beam’s path be visible?

(a) Only A
(b) Only B
(c) B and C
(d) A, B and C

Answer: (c) B and C

Explanation: The Tyndall effect occurs in suspensions (chalk water) and colloids (milk water). Salt water is a true solution — its particles are too small to scatter light, so the beam is invisible.

Question 3: Which of the following correctly classifies the given mixtures?
Ht = heterogeneous
Hm = homogeneous

(a) Air — Ht, Milk — Ht, Sugar solution — Hm, Smoke — Hm
(b) Brass — Ht, Fog — Ht, Vinegar — Ht, Muddy water — Hm
(c) Copper sulfate solution — Hm, Salt solution — Hm, Milk — Hm, Bronze — Hm
(d) Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm

Answer: (iv) Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm

Explanation:

• Muddy water — heterogeneous (suspension)
• Milk — heterogeneous (colloid)
• Blood — heterogeneous (colloid)
• Brass — homogeneous (alloy/solution of metals)

Question 4: Chalk powder is added to water and stirred. Which of the following will be observed?

(a) Chalk dissolves completely — no residue on filter paper
(b) Chalk dissolves partially — some residue on filter paper
(c) Chalk does not dissolve — residue left on filter paper
(d) Chalk disappears and no Tyndall effect is seen

Answer: (c) Chalk does not dissolve — residue left on filter paper

Explanation: Chalk powder forms a suspension in water. Its particles are large, do not dissolve, remain visible, and are trapped by filter paper.

Question 5: Which property is used to distinguish a colloid from a true solution?

(a) Colour
(b) Tyndall effect
(c) Taste
(d) Density

Answer: (b) Tyndall effect

Explanation: A colloid scatters light — making the beam’s path visible (Tyndall effect). A true solution does not scatter light. This is the simplest and most reliable test to distinguish the two.

Assertion–Reason Questions

Instructions: Choose the correct option —

• (i) Both A and R are true, and R is the correct explanation of A
• (ii) Both A and R are true, but R is NOT the correct explanation of A
• (iii) A is true, but R is false
• (iv) A is false, but R is true

Question 1:
Assertion (A): A solution always remains homogeneous.
Reason (R): The solute particles in a solution are smaller than 1 nm and remain uniformly distributed.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Because solute particles are molecular-level tiny (< 1 nm), they cannot settle or separate — keeping the solution permanently homogeneous.

Question 2:
Assertion (A): Milk is classified as a heterogeneous mixture.
Reason (R): Milk particles are visible to the naked eye and settle on standing.

Answer: (iii) A is true, but R is false.

Explanation: Milk is heterogeneous (it is a colloid) — but its particles are NOT visible to the naked eye and do NOT settle on standing. That is what makes it a colloid, not a suspension.

Question 3:
Assertion (A): Sand and water is a heterogeneous mixture.
Reason (R): Sand particles do not dissolve and remain visible, settling over time.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Sand does not dissolve in water — its particles are visible and settle at the bottom when left undisturbed. This non-uniform composition makes it heterogeneous.

Question 4:
Assertion (A): The Tyndall effect is observed in salt water.
Reason (R): Salt water contains particles that scatter light.

Answer: (iv) A is false, but R is true.

Explanation: Salt water does NOT show the Tyndall effect because its particles are too small (< 1 nm). However, R is technically true as a general statement — particles do scatter light, but only when they are large enough (colloid or suspension range).

Question 5:
Assertion (A): Blood is neither a solution nor a true suspension.
Reason (R): Blood is a colloid — its particles are medium-sized and do not settle.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Blood is a colloid — red blood cells and other particles range between 1–1000 nm. They are uniformly dispersed and do not settle, which is why blood behaves consistently throughout the body.

Fill in the Blanks

Question 1:A well-stirred mixture of sugar and water is equally sweet from the first to the last sip. Such a mixture is called a __________.

Answer: homogeneous mixture (or solution)

Explanation: Uniform composition throughout — no matter where you taste it, the sweetness is identical. This is the defining property of a homogeneous mixture.

Question 2: The scattering of light by particles in a mixture is known as the __________ effect.

Answer: Tyndall

Explanation: Named after scientist John Tyndall, this effect makes the path of a light beam visible when passed through a colloid or suspension.

Question 3: In a heterogeneous mixture, the particles of the undissolved substance are __________ in size than the particles present in a solution.

Answer: larger

Explanation: This is why suspension particles are visible and can be filtered — they are much larger (> 1000 nm) than solution particles (< 1 nm).

Question 4: A mixture of sand and water, when left undisturbed, will show __________ of sand particles at the bottom.

Answer: settling (or sedimentation)

Explanation: Sand forms a suspension — its large, heavy particles are pulled down by gravity when the mixture is not disturbed.

Question 5: Colloids have particle sizes ranging from __________ to __________ nm.

Answer: 1 to 1000 nm

Explanation: This medium particle size is what makes colloids unique — large enough to scatter light (Tyndall effect) but small enough to not settle or be filtered.

True or False

Question 1: A solution always remains homogeneous and never separates on standing.

Answer: ✅ True

Explanation: Solute particles in a solution are too tiny (< 1 nm) to settle. They remain permanently and uniformly distributed throughout the solvent.

Question 2: Chalk powder dissolved in water is an example of a homogeneous mixture.

Answer: ❌ False

Correction: Chalk powder in water is a heterogeneous mixture (suspension). Chalk does not dissolve — its particles remain visible and settle over time.

Question 3: The Tyndall effect can be observed in both colloids and suspensions.

Answer: ✅ True

Explanation: Both colloids and suspensions have particles large enough to scatter light. True solutions do NOT show this effect.

Question 4: Milk and water is a heterogeneous mixture because milk particles are visible to the naked eye.

Answer: ❌ False

Correction: Milk is indeed a heterogeneous mixture (colloid), but milk particles are NOT visible to the naked eye. They are medium-sized (1–1000 nm) — too small to see but large enough to scatter light.

Question 5: A suspension can be separated by filtration, but a colloid cannot.

Answer: ✅ True

Explanation: Suspension particles are large enough (> 1000 nm) to be trapped by filter paper. Colloid particles (1–1000 nm) are too small and pass through ordinary filter paper.

5.2 Solutions

Chapter 5 | Exploring Mixtures and Their Separation | Grade 9

1-Mark Questions

Question 1: What is a solute?

Answer: A solute is the substance that gets dissolved in a solvent to form a solution.

Explanation: In sugar water, sugar is the solute since it dissolves completely into the water.

Question 2: What is a solvent?

Answer: A solvent is the substance that dissolves the solute to form a solution.

Explanation: In sugar water, water acts as the solvent because it dissolves the sugar.

Question 3: Define the concentration of a solution.

Answer: Concentration is the amount of solute dissolved in a given amount of solvent or solution.

Explanation: This tells us “how strong” or “how weak” a solution is — like how much salt is in your ORS packet.

Question 4: Who developed the Oral Rehydration Solution (ORS)?

Answer: Dilip Mahalanabis, an Indian paediatrician, developed and implemented ORS.

Explanation: His formula has saved millions of lives worldwide after the WHO popularised it.

Question 5: What does % m/m stand for?

Answer: % m/m stands for mass by mass percentage.

Explanation: It tells how many grams of solute are present in 100 grams of the total solution.

Question 6: What does % m/v stand for?

Answer: % m/v stands for mass by volume percentage.

Explanation: It tells how many grams of solute are present in 100 millilitres of the solution.

Question 7: What does % v/v stand for?

Answer: % v/v stands for volume by volume percentage.

Explanation: It tells how many millilitres of solute are present in 100 millilitres of the solution.

Question 8: Define solubility.

Answer: Solubility is the maximum amount of solute that dissolves in a fixed quantity of solvent (100 mL or 100 g) at a given temperature.

Explanation: Beyond this limit, no more solute can dissolve — the solution becomes saturated.

Question 9: What is a saturated solution?

Answer: A saturated solution cannot dissolve any more solute at a given temperature.

Explanation: Once the saturation point is reached, extra solute simply settles or remains undissolved.

Question 10: A saline drip used in hospitals contains how much sodium chloride?

Answer: Saline solution contains 0.9% m/v sodium chloride in water.

Explanation: This means 0.9 g of salt is present in every 100 mL of solution — a concentration safe for blood.

2-Mark Questions

Question 1: Differentiate between solute and solvent with an example.

Answer:

Explanation: In a sugar solution, sugar is present in a smaller quantity and gets dissolved, making it the solute, while water (a larger quantity) is the solvent.

Question 2: Why can’t we freely add any amount of salt and sugar to water to make ORS?

Answer: ORS requires a specific, fixed concentration of salt and sugar. Changing the amount changes the concentration, and it stops being effective ORS.

Explanation: Too little or too much salt/sugar can make the solution ineffective or even harmful for treating dehydration.

Question 3: Why must farmers mix the correct amount of pesticide with water?

Answer: Farmers must maintain the right concentration of pesticide. Too little pesticide fails to protect crops; too much damages crops, soil, and the environment.

Explanation: This shows why concentration control matters in real-world applications like agriculture.

Question 4: A talcum powder contains 4% m/m zinc oxide. How much zinc oxide is present in 300 g of talcum powder?

Answer: 12 g of zinc oxide

Explanation:$$\text{Mass of zinc oxide} = \frac{4}{100} \times 300 = 12 \text{ g}$$

Question 5: Why is % m/v commonly used in medicines and laboratories?

Answer: % m/v is used because measuring the volume of a liquid is easier than weighing it, especially for liquid medicines.

Explanation: A common example is 5% glucose solution, where doctors need a quick, accurate way to measure dosage by volume.

Question 6: Why does the solubility of a solid in a liquid generally increase with temperature?

Answer: As temperature increases, solvent molecules gain more energy, allowing them to dissolve more solute particles.

Explanation: This is why hot water dissolves sugar faster and in larger amounts than cold water.

Question 7: How does solubility of gases in liquids behave with increasing temperature?

Answer: Solubility of gases in liquids decreases with an increase in temperature.

Explanation: This is why a cold soda bottle has more fizz (dissolved CO₂) than a warm one — heat releases the gas.

Question 8: Vinegar contains 5% v/v acetic acid. What does this mean?

Answer: It means 5 mL of acetic acid is present in every 100 mL of vinegar solution.

Explanation: Vinegar is made by mixing acetic acid (the solute) with water using the volume by volume method since both are liquids.

Question 9: Why is % m/m also used for heterogeneous mixtures like milk powder, even though it is mainly meant for solutions?

Answer: % m/m works for any mixture, homogeneous or heterogeneous, because it simply compares mass of one component to total mass — no dissolving is required.

Explanation: This is why packaged foods label fat, sugar, and protein content using % m/m, regardless of mixture type.

Question 10: If 5 g of glucose is dissolved in water to make 100 mL of solution, calculate the % m/v concentration.

Answer: 5% m/v

Explanation:

3-Mark Questions

Question 1: Calculate the mass by mass percentage if 10 g of salt is dissolved in 90 g of water.

Answer: 10% m/m

Explanation:

$$\text{Total mass of solution} = 10 \text{ g} + 90 \text{ g} = 100 \text{ g}$$

Question 2: A liquid pesticide of 1 mL is mixed with water to form 100 mL pesticide spray. Calculate its % v/v.

Answer: 1% v/v

Explanation:

Question 3: Your mother asks you to mix two tablespoons of orange juice concentrate (each tablespoon = 15 mL) with water to make 150 mL of juice. Calculate the % v/v of concentrate.

Answer: 20% v/v

Explanation:$$\text{Volume of concentrate} = 2 \times 15 \text{ mL} = 30 \text{ mL}$$
$$\% \, v/v = \frac{30}{150} \times 100 = 20\% \, v/v$$

Question 4: Why is the concept of concentration important not just in laboratories but also in everyday life? Give three examples.

Answer: Concentration matters in everyday situations, not just labs:

1. Medicine — saline drips need exact 0.9% m/v concentration to be safe for blood
2. Agriculture — pesticide sprays need correct concentration to protect crops without causing damage
3. Food/Cosmetics — packaged products list precise % m/m for sugar, fat, protein

Explanation: Wrong concentration can make a solution ineffective or even dangerous — whether it’s medicine, food, or farming chemicals.

Question 5: Vinegar contains 5% v/v acetic acid. Glacial acetic acid is 100% acetic acid. How would you prepare vinegar from glacial acetic acid?

Answer: To get 100 mL of vinegar (5% v/v):

• Take 5 mL of glacial acetic acid
• Add water to make the total volume 100 mL

Explanation:
$$\text{Volume of acetic acid needed} = \frac{5}{100} \times 100 \text{ mL} = 5 \text{ mL}$$

Mix 5 mL acetic acid with 95 mL water to get 100 mL of 5% v/v vinegar.

Question 6: Why do we mention temperature when defining solubility?

Answer: Temperature must be mentioned because solubility changes with temperature.

• For solids in liquids → solubility generally increases with temperature
• For gases in liquids → solubility generally decreases with temperature

Explanation: Without specifying temperature, the solubility value would be incomplete and meaningless, since the same substance dissolves differently at different temperatures.

Question 7: Explain why % w/w is commonly used in industries instead of % m/m, even though both are numerically equal.

Answer: Industries use % w/w (weight by weight) because weight and mass are commonly used interchangeably in everyday and industrial language.

Explanation: Even though mass and weight are scientifically different (weight depends on gravity), in regular industrial practice, they are treated as the same — so % m/m = % w/w numerically.

Question 8: Compare the three methods of expressing concentration (% m/m, % m/v, % v/v) based on their use-cases.

Answer:

Explanation: The choice of method depends on what is easier to measure — mass or volume — and the physical state of the solute and solvent.

Question 9: A bottle of glucose intravenous infusion is labelled “5% w/v” and contains 500 mL. How much glucose does it contain?

Answer: 25 g of glucose

Explanation:
$$\text{Mass of glucose} = \frac{5}{100} \times 500 \text{ mL} = 25 \text{ g}$$

This matches the real label shown in Fig. 5.4(b), confirming “Each bottle 500 mL contains: Glucose 25 g.”

Question 10: Explain the difference between solubility of solids and gases in liquids with respect to temperature, using a real-life example for each.

Answer:

Explanation: This opposite behaviour is why hot water is preferred to dissolve solids, but cold temperatures are used to preserve dissolved gases like CO₂ in drinks.

5-Mark Questions

Question 1: Explain the three methods of expressing the concentration of a solution in terms of percentage. Give the formula, one example, and one solved numerical for each.

Answer:

🧪 Three Ways to Express Concentration

A. Mass by Mass Percentage (% m/m or % w/w)

Formula:
$$\% \, m/m = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$$

Used for: Homogeneous mixtures and even heterogeneous ones like milk powder, spice mixtures, and packaged food labels.

Example:
10 g salt in 90 g water → Total mass = 100 g

$$\% \, m/m = \frac{10}{100} \times 100 = 10\% \, m/m$$

B. Mass by Volume Percentage (% m/v or % w/v)

Formula:
$$\% \, m/v = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100$$

Used for: Medicines and laboratory solutions, where volume is easier to measure than weight. Example: 5% glucose solution.

Example:
5 g glucose dissolved to make 100 mL solution

$$\% \, m/v = \frac{5}{100} \times 100 = 5\% \, m/v$$

C. Volume by Volume Percentage (% v/v)

Formula:
$$\% \, v/v = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100$$

Used for: Mixing two miscible liquids, like perfumes, cosmetics, and vinegar.

Example:
1 mL pesticide mixed with water to form 100 mL spray
$$\% \, v/v = \frac{1}{100} \times 100 = 1\% \, v/v$$

Brain Anchor: 🔥 Solid in solid/liquid → m/m. Solid in liquid (easy volume) → m/v. Liquid in liquid → v/v.

Question 2: What is solubility? Draw and explain a solubility curve, and describe how it helps predict the behaviour of solutes at different temperatures.

Answer:

📈 Understanding Solubility Curves

Definition:
Solubility is the maximum amount of solute that dissolves in a fixed quantity of solvent (100 g or 100 mL) at a given temperature. A solution that cannot dissolve more solute is called saturated.

What is a Solubility Curve?

A solubility curve is a graph of solubility (y-axis) versus temperature (x-axis). It shows how much solute (in grams per 100 g water) dissolves at each temperature.

Key Observations from a Solubility Curve

Example (from Compounds A and B)

• At 40°C, Compound B has a solubility of 241 g per 100 g water
• At 60°C, Compound B has solubility of 287 g per 100 g water
• Compound B’s curve rises more steeply than Compound A’s — meaning B’s solubility increases faster with temperature

Why Solubility Curves Matter

1. They help predict how much solute will crystallise out when a saturated solution is cooled
2. They guide industries in choosing the right temperature for dissolving or recovering substances
3. They explain why a saturated solution at high temperature, when cooled, releases excess solute as crystals

Brain Anchor: 🔥 The curve doesn’t just show numbers — it predicts crystals before they even form.

Question 3: A cake recipe uses 75 g sugar, 420 g all-purpose flour, and 5 g sodium hydrogencarbonate. Express the concentration of each component using an appropriate method.

Answer

🎂 Calculating Concentration in the Cake Mixture

Total mass of mixture:
$$75 + 420 + 5 = 500 \text{ g}$$

Using % m/m (Mass by Mass Percentage) since all ingredients are solids:

Verification: 15% + 84% + 1% = 100%

Explanation:

Since this is a solid-solid mixture, % m/m is the most appropriate method — it directly compares the mass of each ingredient to the total mass of the mixture.

Question 4: Three students prepare sugar solutions: Student A (20 g sugar in 80 g water), Student B (20 g sugar in 100 g water), Student C (30 g sugar in 80 g water). Calculate % m/m for each and identify the most concentrated solution.

Answer:

🍬 Comparing Three Sugar Solutions

Most Concentrated Solution:

Student C’s solution is the most concentrated at 27.27% m/m.

Explanation: Student C dissolved the highest mass of sugar (30 g) in the least amount of water (80 g) — giving it the highest sugar-to-solution ratio.

Brain Anchor: 🔥 More solute, less solvent — always means stronger concentration.

Question 5: A brass alloy contains 70% copper by mass. Calculate the quantities of copper and zinc present in 120 g of brass. Also, explain why % m/m is suitable here.

Answer:

🔧 Calculating Copper and Zinc in Brass

Given: Brass = 70% copper by mass, Total mass = 120 g

Step 1: Calculate the mass of copper
$$\text{Mass of copper} = \frac{70}{100} \times 120 = 84 \text{ g}$$

Step 2: Calculate the mass of zinc

Since brass = copper + zinc, zinc percentage = 100% − 70% = 30%
$$\text{Mass of zinc} = \frac{30}{100} \times 120 = 36 \text{ g}$$

Verification: 84 g + 36 g = 120 g

Why % m/m is Suitable

% m/m is ideal here because:

1. Brass is a solid alloy — both components are solids, so mass comparison makes sense
2. Volume measurement would be impractical for solid metals
3. Industries commonly express alloy composition using mass percentage

Explanation
Brass is made of approximately 80% copper and 20% zinc, typically, but this question uses a custom 70-30 ratio to test the % m/m calculation skill.

MCQs (Multiple Choice Questions)

Question 1: In a solution, the substance that gets dissolved is called the:

(a) Solvent
(b) Solute
(c) Concentration
(d) Mixture

Answer: (b) Solute

Explanation: The solute is the substance present in a smaller quantity that dissolves into the solvent to form a solution.

Question 2: Which method of expressing concentration is most suitable for vinegar?

(a) % m/m
(b) % m/v
(c) % v/v
(d) None of these

Answer: (c) % v/v

Explanation: Vinegar is a mixture of two miscible liquids — acetic acid and water. % v/v is used specifically for liquid-liquid mixtures.

Question 3: A saline drip used in hospitals has a concentration of:

(a) 5% m/v
(b) 0.9% m/v
(c) 10% m/m
(d) 1% v/v

Answer: (b) 0.9% m/v

Explanation: Saline solution contains 0.9 g of sodium chloride in 100 mL of solution — safe for blood and used to replace lost body fluids.

Question 4: As temperature increases, the solubility of a gas in a liquid generally:

(a) Increases
(b) Decreases
(c) Remains constant
(d) First increases then decreases

Answer: (b) Decreases

Explanation: Unlike solids, gases dissolve less in liquids as temperature rises — this is why warm soda goes flat faster than cold soda.

Question 5: Which of the following is the correct formula for mass by volume percentage?

(a) (Mass of solute / Mass of solution) × 100
(b) (Mass of solute / Volume of solution) × 100
(c) (Volume of solute / Volume of solution) × 100
(d) (Volume of solute / Mass of solution) × 100

Answer: (b) (Mass of solute / Volume of solution) × 100

Explanation: % m/v compares the mass of solute (grams) to the volume of the total solution (mL) — commonly used for medicines like glucose solutions.

Question 6: A solution that cannot dissolve any more solute at a given temperature is called:

(a) Dilute solution
(b) Concentrated solution
(c) Saturated solution
(d) Unsaturated solution

Answer: (c) Saturated solution

Explanation: Once a solution reaches its solubility limit at a particular temperature, it becomes saturated — no more solute can dissolve.

Question 7: Which scientist developed the Oral Rehydration Solution (ORS)?

(a) John Tyndall
(b) Dilip Mahalanabis
(c) C.V. Raman
(d) Homi Bhabha

Answer: (b) Dilip Mahalanabis

Explanation: This Indian paediatrician’s ORS formula has saved millions of lives globally, after being popularised by the WHO.

Question 8: 5 g of glucose dissolved in water to make 100 mL solution has a concentration of:

(a) 5% m/m
(b) 5% v/v
(c) 5% m/v
(d) 50% m/v

Answer: (c) 5% m/v

Explanation: Since mass (5 g) is compared to volume (100 mL) of the final solution, this is correctly expressed as % m/v.

Question 9: Which of the following statements about solubility is correct?

(a) The solubility of solids in liquids always decreases with temperature
(b) The solubility of gases in liquids always increases with temperature
(c) Solubility of solids in liquids generally increases with temperature
(d) Solubility does not depend on temperature at all

Answer: (c) Solubility of solids in liquids generally increases with temperature

Explanation: This is why hot water dissolves more sugar or salt than cold water — increased temperature provides more energy for dissolution.

Question 10: Which method of concentration is commonly used to label packaged foods showing salt, sugar, or protein content?

(a) % v/v
(b) % m/v
(c) % m/m
(d) Molarity

Answer: (c) % m/m

Explanation: Packaged food labels (like milk powder) typically use % m/m since it works for both solid-solid and solid-liquid mixtures.

Assertion–Reason Questions

Instructions: Choose the correct option —

• (i) Both A and R are true, and R is the correct explanation of A
• (ii) Both A and R are true, but R is NOT the correct explanation of A
• (iii) A is true, but R is false
• (iv) A is false, but R is true

Question 1:
Assertion (A): We cannot freely add any amount of salt and sugar to water to make ORS.
Reason (R): ORS requires a specific, fixed concentration of salt and sugar in water.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Changing the specified concentration of salt or sugar means the solution is no longer effective ORS, even though it remains a salt-sugar solution.

Question 2:
Assertion (A): % m/v is commonly used in medicines.
Reason (R): Measuring the volume of a liquid is easier than weighing it.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: This is exactly why medical solutions like glucose IV infusions are labelled in % m/v — volume is quicker to measure in clinical settings.

Question 3:
Assertion (A): The solubility of gases in liquids increases with rising temperature.
Reason (R): Higher temperature provides more energy for gas molecules to dissolve.

Answer: (iv) A is false, but R is true.

Explanation: A is incorrect — gas solubility actually decreases with rising temperature (not increases). R describes a mechanism that applies to solids, not gases.

Question 4:
Assertion (A): % v/v is used to express the concentration of vinegar.
Reason (R): Vinegar is formed by mixing two miscible liquids — acetic acid and water.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Since both components are liquids that mix completely, the volume-to-volume ratio is the natural way to express vinegar’s concentration.

Question 5:
Assertion (A): % m/m and % w/w are numerically different from each other.
Reason (R): Mass and weight are scientifically different physical quantities.

Answer: (iv) A is false, but R is true.

Explanation: Although mass and weight are scientifically different, they are used interchangeably in industries — making % m/m numerically equal to % w/w. So A is false.

Question 6:
Assertion (A): A saturated solution at a given temperature can dissolve more solute if heated further.
Reason (R): Solubility of solids generally increases with temperature.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Since solubility of solids rises with temperature, a solution saturated at a lower temperature can dissolve additional solute once heated further.

Question 7:
Assertion (A): Talcum powder’s zinc oxide content is expressed using % m/m.
Reason (R): Talcum powder is a heterogeneous solid mixture, and % m/m can be used for such mixtures too.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: % m/m isn’t limited to solutions — it applies to heterogeneous solid mixtures as well, like talcum powder containing zinc oxide as an antiseptic.

Question 8:
Assertion (A): Pesticide sprays must be mixed in correct concentration with water.
Reason (R): Too little pesticide protects crops better than too much.

Answer: (iii) A is true, but R is false.

Explanation: A is correct — concentration matters for pesticide sprays. But R is false: too little pesticide does NOT protect crops better; in fact, it fails to protect crops adequately, while too much damages crops and soil.

Question 9:
Assertion (A): A solubility curve plots solubility against temperature.
Reason (R): Solubility curves help predict how much solute will crystallize when a saturated solution is cooled.

Answer: (ii) Both A and R are true, but R is NOT the correct explanation of A.

Explanation: A correctly defines what a solubility curve is (a plot of solubility vs temperature). R is a true but separate fact about an application of solubility curves — it doesn’t explain what the curve itself represents.

Question 10:
Assertion (A): The concentration of ORS can be changed slightly without affecting its effectiveness.
Reason (R): Concentration is the amount of solute dissolved in a given amount of solvent or solution.

Answer: (iv) A is false, but R is true.

Explanation: A is false — even small changes in ORS concentration can make it ineffective or unsafe. R is a true, general definition of concentration but does not support A.

Fill in the Blanks

Question 1: The substance that gets dissolved in a solution is called the __________.

Answer: solute

Explanation: It is present in a smaller quantity and gets dissolved by the solvent.

Question 2: The amount of solute dissolved in a given amount of solvent or solution is called __________.

Answer: concentration

Explanation: This term tells us how strong or weak a solution is.

Question 3: % m/v tells us the number of grams of solute present in __________ mL of solution.

Answer: 100

Explanation: This standard reference volume makes it easy to compare concentrations of different solutions.

Question 4: Vinegar contains __________ % v/v of acetic acid.

Answer: 5

Explanation: This means 5 mL of acetic acid is present in every 100 mL of vinegar.

Question 5: A solution that cannot dissolve more solute at a given temperature is called a __________ solution.

Answer: saturated

Explanation: Once saturation is reached, any additional solute will remain undissolved.

Question 6: The solubility of __________ in liquids generally decreases with an increase in temperature.

Answer: gases

Explanation: This explains why warm carbonated drinks lose their fizz faster than cold ones.

Question 7: A graph of solubility versus temperature is called a __________.

Answer: solubility curve

Explanation: It helps predict how solubility changes as temperature rises or falls.

Question 8: Saline solution used in hospitals contains __________ % m/v sodium chloride.

Answer: 0.9

Explanation: This concentration is safe for blood and helps restore lost body fluids.

Question 9: __________ developed the Oral Rehydration Solution (ORS) formula.

Answer: Dilip Mahalanabis

Explanation: His work as an Indian paediatrician revolutionised rehydration therapy worldwide.

Question 10: The commonly used unit in industries, numerically equal to % m/m, is __________.

Answer: % w/w (weight by weight percentage)

Explanation: Since weight and mass are used interchangeably in industry, both percentages are numerically the same.

True or False

Question 1: A solute is always present in a larger quantity than the solvent.

Answer: ❌ False

Correction: The solute is present in a smaller quantity, while the solvent is present in a larger quantity in a solution.

Question 2: Changing the volume of water while keeping salt and sugar amounts fixed will still produce proper ORS.

Answer: ❌ False

Correction: Changing the volume of water changes the concentration, so the result will not be proper ORS — it becomes just a salt-sugar solution.

Question 3: % v/v is used when two miscible liquids are mixed.

Answer: ✅ True

Explanation: Examples include vinegar, perfumes, and cosmetics — all involve liquid-liquid mixtures.

Question 4: Solubility of a solid solute in a liquid solvent generally decreases with an increase in temperature.

Answer: ❌ False

Correction: Solubility of solids in liquids generally increases with temperature — it is gases whose solubility decreases.

Question 5: % m/m can be used for both homogeneous and heterogeneous mixtures.

Answer: ✅ True

Explanation: It applies to solutions as well as mixtures like milk powder and spice mixtures, since it only compares mass.

Question 6: A saturated solution can always dissolve more solute regardless of temperature.

Answer: ❌ False

Correction: A saturated solution cannot dissolve more solute at that specific temperature, but it might dissolve more if the temperature is increased.

Question 7: Too much pesticide can damage crops, soil, and the environment.

Answer: ✅ True

Explanation: This is why correct concentration is essential — both too little and too much pesticide cause problems.

Question 8: Numerically, % m/m and % w/w are different from each other.

Answer: ❌ False

Correction: % m/m and % w/w are numerically equal, since mass and weight are used interchangeably in industries.

Question 9: A 5% glucose solution means 5 mL of glucose is present in 100 g of solution.

Answer: ❌ False

Correction: A 5% glucose solution (% m/v) means 5 g of glucose is present in 100 mL of solution — not 5 mL in 100 g.

Question 10: Understanding concentration is useful only in chemistry laboratories.

Answer: ❌ False

Correction: Concentration is essential in everyday life too — in medicine, agriculture, food, cosmetics, and even making a simple cup of tea.

5.3 Methods of Separation of Homogeneous Mixtures

Chapter 5 | Exploring Mixtures and Their Separation | Grade 9

1-Mark Questions

Question 1: What is crystallisation?

Answer: Crystallisation is the process of forming crystals from a saturated solution.

Explanation: This technique separates a pure solid from a solution by allowing it to form regular, geometric crystal shapes.

Question 2: What is distillation?

Answer: Distillation is a method of separating a homogeneous mixture of two miscible liquids by heating until the liquid with the lower boiling point vaporises, then cooling the vapour back to liquid.

Explanation: It works because different liquids have different boiling points.

Question 3: What is paper chromatography?

Answer: Paper chromatography is a method of separating components of a mixture based on differences in their interaction with the solvent and paper.

Explanation: It is widely used to separate dyes, inks, and pigments.

Question 4: What is a crystal?

Answer: A crystal is a solid made up of particles arranged in a regular geometric pattern.

Explanation: Examples include rock salt, sugar (mishri), snowflakes, and frost.

Question 5: What is the boiling point of acetone?

Answer: Acetone boils at about 56°C.

Explanation: This is much lower than water’s boiling point (100°C), which is why acetone and water can be separated by distillation.

Question 6: Name the traditional Indian distillation method used in Kannauj to make perfume.

Answer: The Deg-Bhapka method.

Explanation: This method is used to create Mitti ka Ittar (earthy fragrance), a natural perfume capturing the smell of first rain.

Question 7: What is fractional distillation?

Answer: Fractional distillation separates components of a mixture with relatively small differences (less than 25°C) in their boiling points.

Explanation: It is used in petroleum refineries to separate crude oil into petrol, diesel, kerosene, etc.

Question 8: What word origin gives “chromatography” its meaning?

Answer: It comes from Greek words chroma (colour) and graphein (to write), meaning “writing with colour.”

Explanation: It was originally used to separate coloured substances such as dyes and inks.

Question 9: What minimum difference in boiling points is needed for simple distillation to work effectively?

Answer: A minimum boiling point difference of about 25°C.

Explanation: Below this difference, simple distillation is not effective, and fractional distillation is used instead.

Question 10: In paper chromatography, what carries the ink components up the paper?

Answer: The solvent (water) carries the ink components up the paper.

Explanation: As water rises through capillary action, it separates the ink into different colour spots based on how fast each component moves.

2-Mark Questions

Question 1: On what principle is purification by crystallisation based?

Answer: Crystallization is based on the differences in solubility of a substance at different temperatures.

Explanation: As a hot saturated solution cools, solubility decreases, forcing excess solute to separate out as pure crystals.

Question 2: Why is sulfuric acid added during the preparation of copper sulfate crystals?

Answer: Sulfuric acid helps in making pure crystals by preventing unwanted reactions.

Explanation: Without it, impurities or side reactions could interfere with crystal formation, but note it’s required only for some salts, not all.

Question 3: Why can a mixture of acetone and water be separated by distillation?

Answer: Their boiling points differ sufficiently — acetone boils at 56°C and water at 100°C.

Explanation: This large gap allows acetone to vaporise completely before water starts forming significant vapour.

Question 4: What happens to the lower-boiling liquid during distillation after it vaporises?

Answer: The vapour passes through a condenser, where it is cooled (usually by circulating water or air) and condensed back into a pure liquid.

Explanation: This pure liquid, called the distillate, is collected separately from the original mixture.

Question 5: Why should the water level be kept below the ink spot at the start of paper chromatography?

Answer: If water touches the ink spot directly, the ink would dissolve immediately into the water instead of rising through the paper by capillary action.

Explanation: The lower-positioned spot ensures the solvent travels upward through the paper, properly separating the components.

Question 6: Why does camphor separate from sand by sublimation, even though this falls under chromatography’s chapter context of separation methods? (Comparison-type question)

Answer: This question belongs to heterogeneous mixture separation (5.4.2), not 5.3 — but for homogeneous mixtures, crystallization, distillation, and chromatography are used instead, since the components are uniformly mixed at a molecular level.

Explanation: This highlights why Section 5.3 methods (crystallization, distillation, chromatography) apply specifically to homogeneous mixtures, unlike sublimation which separates heterogeneous solid mixtures.

Question 7: What can crystallization be used for besides separating two solids?

Answer: Crystallization can also be used for purification of solids.

Explanation: When new compounds are prepared, they often come with unwanted impurities — crystallization helps separate the pure substance from these impurities.

Question 8: Name two natural locations where large crystal deposits can be found.

Answer: Mawsmai Cave in Sohra (Cherrapunji) and various mines and caves within the Earth’s crust.

Explanation: Quartz is another beautiful example of a naturally occurring crystal found in such locations.

Question 9: What is the purpose of a watch glass in the crystallization activity?

Answer: The watch glass is used to cover the filtered solution (preventing dust/contamination) and later to hold and dry the formed crystals.

Explanation: It plays a dual role — protecting the solution during cooling and acting as a drying surface for finished crystals.

Question 10: What ancient Indian methods were used for crystallising salt from seawater?

Answer: The panga salt method (boiling concentrated sea brines) and the karkatch salt method (evaporation of seawater).

Explanation: Different methods produced salt crystals of different sizes, showing early Indian innovation in crystallisation techniques.

3-Mark Questions

Question 1: Describe the steps to prepare copper sulfate crystals using crystallisation.

Answer:

🔬 Steps in Crystallisation

1. Dissolve copper sulfate in water with a drop of dilute sulfuric acid; heat gently while stirring until saturated
2. Filter the hot solution to remove insoluble impurities
3. Cool the filtrate slowly without disturbing it, allowing crystals to form
4. Filter, rinse, and dry the crystals on a watch glass

Explanation: Slow cooling gives particles enough time to arrange into larger, well-shaped, shiny crystals.

Question 2: Why does cooling a hot saturated solution slowly give better crystals than cooling it rapidly?

Answer: Slow cooling gives particles enough time to arrange themselves into a regular geometric pattern, forming larger, well-shaped crystals.

Explanation: Rapid cooling (e.g., in ice-cold water) doesn’t allow this orderly arrangement, resulting in smaller, poorly-formed crystals.

Question 3: Describe the basic setup and process of distillation with reference to the distillation flask, condenser, and conical flask.

Answer:

🧪 Distillation Setup

Process: The mixture is heated → lower-boiling liquid vaporises → vapour passes through condenser → cools and condenses → pure liquid collected in conical flask.

Explanation: The solid or higher-boiling liquid remains behind in the distillation flask.

Question 4: Explain the steps of Activity 5.5 (paper chromatography) used to separate ink components.

Answer

📝 Paper Chromatography Steps

1. Take a 3 cm wide strip of chromatographic paper; draw a pencil line 2 cm from the bottom
2. Mark an ink spot at the centre of the line
3. Place a thin layer of water at the bottom of a container
4. Dip the paper strip vertically so its lower end touches water, keeping water level below the spot
5. As water rises, the ink separates into different colour spots

Explanation: Components separate because they interact differently with the solvent and paper, moving at different speeds.

Question 5: A student needs to separate two miscible liquids A (boiling point 60°C) and B (boiling point 90°C). Which method should be used, and why?

Answer: Distillation should be used.

Explanation: The difference in boiling points is 90°C − 60°C = 30°C, which is more than 25°C, satisfying the minimum requirement for simple distillation. Liquid A (lower boiling point) will vaporise first and be collected as distillate.

Question 6: Explain how perfumes are extracted in Kannauj using the Deg-Bhapka method.

Answer: The Deg-Bhapka method is a traditional distillation technique passed down through generations in Kannauj, Uttar Pradesh — known as the “perfume capital of India.”

Explanation: This method captures the earthy fragrance released after the first rain and turns it into a natural perfume called Mitti ka Ittar. It is in great demand both in India and internationally.

Question 7: What is fractional distillation, and how is it used in petroleum refining?

Answer: Fractional distillation separates components with boiling point differences of less than 25°C.

Explanation: In petroleum refineries, crude oil is heated in a furnace and separated into fractions like petroleum gas, petrol, kerosene, diesel, lubricating oil, and bitumen — each collected at different heights in the fractionating column based on boiling point.

Question 8: Why is filtration of the hot saturated solution necessary before crystallization?

Answer: Filtration removes insoluble impurities present in the solution.

Explanation: If impurities aren’t removed before cooling, they could get trapped within the forming crystals, resulting in impure crystal products.

Question 9: Compare the boiling points and conditions needed for simple distillation versus fractional distillation.

Answer

Explanation: The smaller the boiling point gap, the more precise the separation technique needed — hence, fractional distillation for crude oil’s closely-boiling components.

Question 10: Why is it important to recover the solvent in some separation processes, and which method achieves this?

Answer: Sometimes we need both the solute and the solvent — not just the dissolved substance. Distillation achieves this by recovering the solvent in pure liquid form.

Explanation: Unlike evaporation (where the solvent disappears as vapour into the air), distillation collects and condenses the vapour, allowing us to recover the solvent for reuse.

5-Mark Questions

Question 1: Describe the complete process of crystallization with the help of Activity 5.3 (preparation of copper sulfate crystals). Include safety precautions and explain the underlying principle.

Answer:

🔬 Crystallization — Full Process

Underlying Principle: Crystallization is based on the difference in solubility of a substance at different temperatures. As a hot saturated solution cools, its solubility decreases, forcing the excess solute to separate out as crystals.

Step-by-Step Process (Activity 5.3)

Safety Precautions

• Copper sulfate is toxic — perform under adult supervision; never touch with bare hands
• Sulfuric acid should be added by the teacher only — handle with extreme care

Why Slow Cooling Matters

Slow cooling allows particles time to arrange into a regular geometric pattern, producing larger, shiny, well-shaped crystals. Rapid cooling produces smaller, poorly-formed crystals.

Brain Anchor: 🔥 Patience makes perfect crystals — rush it, and you get a mess.

Question 2: Explain the process of distillation with a labelled diagram description. Why does it work for separating acetone and water? Mention one real-world application.

Answer

🧪 Distillation — Complete Explanation

Definition: Distillation separates a homogeneous mixture of two miscible liquids by heating until the lower-boiling liquid vaporises, then cooling the vapour back into liquid.

Labelled Setup Components

Why It Works for Acetone-Water

The 44°C gap (well above the minimum 25°C requirement) ensures acetone vaporises completely before water forms significant vapour, allowing clean separation.

Real-World Application

Petroleum refining uses a related technique (fractional distillation) to separate crude oil into petrol, diesel, and kerosene. Traditional perfume-making in Kannauj uses the Deg-Bhapka distillation method to create Mitti ka Ittar.

Brain Anchor: 🔥 Heat separates, cold collects — that’s distillation in two words.

Question 3: Explain paper chromatography in detail. Include the activity steps, the principle behind separation, and one application beyond ink separation.

Answer

📝 Paper Chromatography — Detailed Explanation

Definition: Paper chromatography separates mixture components using differences in their interaction with the solvent and the paper.

Activity 5.5 Steps

1. Take a 3 cm strip of chromatographic (or filter) paper; draw a pencil line 2 cm from the bottom
2. Mark a black sketch pen spot at the centre of the line
3. Place a thin layer of water at the bottom of a gas jar or beaker
4. Dip the strip vertically so the lower end touches water, with water level below the ink spot
5. Observe as water rises — ink separates into different colour spots

Principle Behind Separation

Components that interact less with paper (more with solvent) travel faster and farther.

Beyond Ink — Other Applications

• Separating pigments in green spinach leaf extract
• Separating coloured pigments from flower petals
• Finding components in food colours or coloured mouth fresheners (like fennel seeds)

Note: Water isn’t a solvent in every case — sometimes alcohol or a solvent mixture is required.

Brain Anchor: 🔥 Different speeds, different destinies — that’s how chromatography separates a mixture.

Question 4: Compare crystallization, distillation, and paper chromatography on the basis of principle, type of mixture separated, and one example each.

Answer

⚖️ Comparing the Three Separation Methods

When to Choose Which Method

• Crystallization → when you need a pure solid from a solution
• Distillation → when you need to recover a liquid solvent or separate liquids
• Paper chromatography → when you need to separate closely related coloured substances

Brain Anchor: 🔥 Cool it for crystals, heat it for liquids, soak it for colours.

Question 5: A mixture of sand, common salt, and naphthalene needs to be separated using different separation techniques. Explain the correct sequence and reasoning for each step.

Answer

🧪 Separating Sand + Salt + Naphthalene

Given Mixture Composition:

• Naphthalene — sublimable solid
• Common salt — soluble in water
• Sand — insoluble, non-sublimable solid

Correct Sequence of Separation

Reasoning for Each Step

1. Sublimation first — naphthalene transforms directly from solid to vapour, easily separated using an inverted funnel setup, leaving sand and salt mixture behind
2. Dissolving + filtration — salt dissolves in water (forms a solution) while sand does not, so filtration separates insoluble sand from the salt solution
3. Evaporation/crystallization — heating the filtrate removes water, leaving behind pure crystallized salt

Brain Anchor: 🔥 Sublime it, dissolve it, then crystallise it — three states, three solutions.

MCQs (Multiple Choice Questions)

Question 1: Crystallization is based on the principle of:

(a) Difference in boiling points
(b) Difference in solubility at different temperatures
(c) Difference in particle size
(d) Difference in density

Answer: (b) Difference in solubility at different temperatures

Explanation: As a hot saturated solution cools, solubility decreases, causing excess solute to separate out as pure crystals.

Question 2: Acetone and water can be separated by distillation because:

(a) They have the same boiling point
(b) Acetone is denser than water
(c) Their boiling points differ sufficiently (56°C vs 100°C)
(d) Water evaporates faster than acetone

Answer: (c) Their boiling points differ sufficiently (56°C vs 100°C)

Explanation: This 44°C difference allows acetone to vaporise completely before significant water vapour forms.

Question 3: What is the minimum boiling point difference required for effective simple distillation?

(a) 10°C
(b) 15°C
(c) 25°C
(d) 50°C

Answer: (c) 25°C

Explanation: Below this difference, fractional distillation (not simple distillation) is used to achieve proper separation.

Question 4: In paper chromatography, the word “chromatography” comes from Greek words meaning:

(a) Writing with light
(b) Writing with colour
(c) Separating with water
(d) Mixing with colour

Answer: (b) Writing with colour

Explanation: From chroma (colour) and graphein (to write) — describing how dyes and inks are separated and visualised.

Question 5: Why is sulfuric acid added while preparing copper sulfate crystals?

(a) To increase the temperature
(b) To prevent unwanted reactions and ensure pure crystals
(c) To dissolve the sand
(d) To change the colour of crystals

Answer: (b) To prevent unwanted reactions and ensure pure crystals

Explanation: It is required only for some salts, helping avoid side reactions during crystallization.

Question 6: Fractional distillation is used when boiling point differences are:

(a) More than 25°C
(b) Less than 25°C
(c) Exactly 25°C
(d) More than 100°C

Answer: (b) Less than 25°C

Explanation: This precise method is essential for separating closely-boiling fractions, such as those found in crude petroleum.

Question 7: Which Indian traditional method is used to extract perfume in Kannauj?

(a) Sublimation method
(b) Deg-Bhapka method
(c) Centrifugation method
(d) Chromatography method

Answer: (b) Deg-Bhapka method

Explanation: This traditional distillation technique has been passed down for generations to create the perfume Mitti ka Ittar.

Question 8: In Activity 5.5, why is the water level kept below the ink spot initially?

(a) To prevent the ink from dissolving directly into water
(b) To make the experiment faster
(c) To avoid wasting water
(d) To keep the paper dry

Answer: (a) To prevent the ink from dissolving directly into water

Explanation: If water touches the spot directly, the ink would dissolve immediately rather than rising properly through the paper.

Question 9: Which of these is NOT a product of fractional distillation of crude petroleum?

(a) Petrol
(b) Diesel
(c) Sugar crystals
(d) Bitumen

Answer: (c) Sugar crystals

Explanation: Sugar crystals are obtained through crystallization, not fractional distillation. Petroleum gas, petrol, kerosene, diesel, lubricating oil, and bitumen are all petroleum fractions.

Question 10: A hot, saturated solution cooled rapidly in ice-cold water will produce:

(a) Larger, well-formed crystals
(b) Smaller, less well-formed crystals
(c) No crystals at all
(d) Only liquid, no solid

Answer: (b) Smaller, less well-formed crystals

Explanation: Rapid cooling doesn’t give particles enough time to arrange into a proper geometric pattern, unlike slow cooling at room temperature.

Assertion–Reason Questions

Instructions: Choose the correct option —

• (i) Both A and R are true, and R is the correct explanation of A
• (ii) Both A and R are true, but R is NOT the correct explanation of A
• (iii) A is true, but R is false
• (iv) A is false, but R is true

Question 1:
Assertion (A): Crystallization is used to separate two solids dissolved in the same solvent.
Reason (R): Crystallization is based on differences in solubility of substances at different temperatures.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Since different solutes have different solubility curves, cooling causes the less-soluble compound (present in smaller quantity) to crystallize out first.

Question 2:
Assertion (A): A mixture of acetone and water can be separated by distillation.
Reason (R): Acetone and water have boiling points that are very close to each other.

Answer: (iii) A is true, but R is false.

Explanation: A is correct — distillation does work here. But R is false — their boiling points (56°C and 100°C) differ by 44°C, which is a large difference, not a close one.

Question 3:
Assertion (A): Distillation can be used to separate two liquids even when they have the same boiling point.
Reason (R): Distillation depends on differences in boiling points to separate liquids.

Answer: (iv) A is false, but R is true.

Explanation: A is false — distillation cannot separate liquids with the same boiling point, since neither liquid would vaporise preferentially. R is a true general principle of distillation.

Question 4:
Assertion (A): In paper chromatography, the solvent level should be above the sample spot at the start of the experiment.
Reason (R): This allows the solvent to carry the sample components upward through the paper.

Answer: (iv) A is false, but R is true.

Explanation: A is false — the water level should be below the spot initially, not above (otherwise the ink dissolves directly). R is generally true about how solvents move samples, but doesn’t support the incorrect setup in A.

Question 5:
Assertion (A): Evaporation and crystallization are the same processes.
Reason (R): Both processes involve heating a solution to remove the solvent.

Answer: (iv) A is false, but R is true.

Explanation: A is false — crystallization produces structured crystals through controlled cooling of a saturated solution, while evaporation simply removes solvent, often leaving an amorphous solid. R describes evaporation’s general process but doesn’t make A correct.

Question 6:
Assertion (A): Slow cooling of a saturated solution produces larger, well-shaped crystals.
Reason (R): Slow cooling gives particles enough time to arrange in a regular geometric pattern.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: This is exactly why crystallization activities recommend allowing solutions to cool undisturbed rather than rapidly.

Question 7:
Assertion (A): Fractional distillation is used in petroleum refineries.
Reason (R): Petroleum components have boiling point differences of more than 25°C.

Answer: (iii) A is true, but R is false.

Explanation: A is correct. R is false — fractional distillation is specifically used because petroleum components have boiling point differences of less than 25°C, requiring a more precise separation method.

Question 8:
Assertion (A): Sulfuric acid is required for crystallization of all salts.
Reason (R): Sulfuric acid prevents unwanted reactions during crystal formation.

Answer: (iv) A is false, but R is true.

Explanation: A is false — sulfuric acid is required for crystallization of only some salts, not all. R correctly describes the function of sulfuric acid when it is used.

Question 9:
Assertion (A): Paper chromatography can only use water as a solvent.
Reason (R): Different substances require different solvents, such as alcohol, depending on their solubility properties.

Answer: (iv) A is false, but R is true.

Explanation: A is false — water does not work as a solvent in every case. R correctly explains why alternative solvents like alcohol may sometimes be needed.

Question 10:
Assertion (A): The Deg-Bhapka method is a traditional Indian crystallization technique.
Reason (R): It is used to extract earthy fragrance and turn it into perfume.

Answer: (iv) A is false, but R is true.

Explanation: A is false — Deg-Bhapka is a distillation method, not crystallisation. R correctly describes its use in creating the perfume Mitti ka Ittar.

Fill in the Blanks

Question 1: The process of forming crystals from a saturated solution is called __________.

Answer: crystallization

Explanation: This technique separates pure solids from solutions using temperature-dependent solubility differences.

Question 2: A solid made up of particles arranged in a regular geometric pattern is called a __________.

Answer: crystal

Explanation: Examples include rock salt, sugar crystals, snowflakes, and frost.

Question 3: Distillation can separate two miscible liquids that differ in boiling point by at least __________°C.

Answer: 25

Explanation: This is the minimum threshold for simple distillation to work effectively.

Question 4: Acetone boils at about __________°C, while water boils at __________°C.

Answer: 56; 100

Explanation: This large boiling point gap allows clean separation of acetone from water through distillation.

Question 5: __________ distillation is used to separate components with boiling point differences of less than 25°C.

Answer: Fractional

Explanation: This technique is essential in petroleum refining to separate closely-boiling fractions.

Question 6: In paper chromatography, the word “chroma” means __________ in Greek.

Answer: colour

Explanation: Combined with graphein (to write), it gives the meaning “writing with colour.”

Question 7: The Deg-Bhapka method is a traditional distillation technique used in __________ to make perfume.

Answer: Kannauj

Explanation: Kannauj, in Uttar Pradesh, is known as the “perfume capital of India.”

Question 8: During crystallization, __________ is added to help prevent unwanted reactions when preparing copper sulfate crystals.

Answer: (dilute) sulfuric acid

Explanation: This acid is required for the crystallisation of only some salts, ensuring pure crystal formation.

Question 9: In distillation, the vapour is cooled in a __________ before being collected as pure liquid.

Answer: condenser

Explanation: The condenser typically uses circulating water or air to cool and condense the vapour.

Question 10: The ancient Indian salt obtained by boiling concentrated sea brines is called __________ salt.

Answer: panga

Explanation: This contrasts with karkatch salt, obtained through seawater evaporation.

True or False

Question 1: Salt can be separated from a salt solution by evaporation or distillation.

Answer: ✅ True

Explanation: Evaporation removes water to leave salt behind, while distillation can additionally recover the water as a pure liquid.

Question 2: Distillation can be used for separation of two liquids even when these have the same boiling point.

Answer: ❌ False

Correction: Distillation cannot separate liquids with the same boiling point — it relies on differences in boiling points to work.

Question 3: In paper chromatography, the solvent level should be above the sample spot at the beginning of the experiment.

Answer: ❌ False

Correction: The solvent (water) level should be below the sample spot at the start, so the ink doesn’t dissolve directly into the water.

Question 4: Evaporation and crystallization are the same processes.

Answer: ❌ False

Correction: These are different processes. Crystallization specifically produces structured crystals through controlled cooling of a saturated solution, while evaporation simply drives off the solvent.

Question 5: Crystallization can be used for purification of solids.

Answer: ✅ True

Explanation: It separates the desired pure substance from unwanted impurities that accompany newly prepared compounds

Question 6: Sulfuric acid is required for the crystallization of all types of salts.

Answer: ❌ False

Correction: Sulfuric acid is required for crystallization of only some salts, not all.

Question 7: Fractional distillation separates components with boiling point differences greater than 25°C.

Answer: ❌ False

Correction: Fractional distillation is used when boiling point differences are less than 25°C — for larger differences, simple distillation works.

Question 8: The Deg-Bhapka method has been used in Kannauj for generations to make perfume.

Answer: ✅ True

Explanation: This traditional distillation method captures earthy fragrance to make the perfume Mitti ka Ittar.

Question 9: Rapid cooling of a hot saturated solution produces larger, well-formed crystals.

Answer: ❌ False

Correction: Rapid cooling produces smaller, less well-formed crystals. Slow cooling at room temperature gives larger, well-shaped crystals.

Question 10: Petroleum gas, petrol, diesel, and bitumen are all obtained through fractional distillation of crude oil.

Answer: ✅ True

Explanation: These are different fractions separated based on their boiling points during the refining process

5.4 How Can We Separate the Components of Heterogeneous Mixtures?

Chapter 5 | Exploring Mixtures and Their Separation | Grade 9

1-Mark Questions

Question 1: What are immiscible liquids?

Answer: Immiscible liquids are liquids that do not mix and form separate layers.

Explanation: Mustard oil and water are a classic example — they never blend into a uniform mixture.

Question 2: Which apparatus is used to separate two immiscible liquids?

Answer: A separating funnel is used.

Explanation: It allows the denser liquid (like water) to be drained out from the bottom through a stopcock, leaving the lighter liquid (like oil) behind.

Question 3: What is sublimation?

Answer: Sublimation is the process where a solid changes directly into vapour (below its melting point) without passing through the liquid state.

Explanation: Camphor is a common example used to demonstrate this in labs.

Question 4: What is deposition?

Answer: Deposition is the process where vapours cool and change directly back into a solid, without becoming a liquid first.

Explanation: This is the reverse of sublimation, seen when camphor vapour solidifies on a cold funnel’s inner wall.

Question 5: What is a suspension?

Answer: A suspension is a heterogeneous mixture in which solid particles do not dissolve but remain suspended throughout the medium.

Explanation: Sand in water is a typical example — particles remain visible and settle over time.

Question 6: What is centrifugation?

Answer: Centrifugation is a process of spinning a mixture at high speed to separate its components based on density.

Explanation: Heavier particles move outward and settle, while lighter liquid remains at the top.

Question 7: What is coagulation?

Answer: Coagulation is the process in which fine suspended particles clump together to form larger particles, called flocs.

Explanation: Alum (fitkari) is commonly used as a coagulant to purify muddy water.

Question 8: Name one coagulant used in water purification.

Answer: Alum (fitkari) is used as a coagulant.

Explanation: It causes fine suspended particles in muddy water to clump together and settle down.

Question 9: What is a colloid?

Answer: A colloid is a mixture in which particle size lies between 1 and 1000 nm — too small to settle but large enough to scatter light.

Question 10: What is an alloy?

Answer: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal.

Explanation: Common examples include brass, bronze, and stainless steel — these cannot be separated by physical methods.

2-Mark Questions

Question 1: Why does mustard oil form the upper layer and water the lower layer in a separating funnel?

Answer: Mustard oil has a lower density than water, so it floats on top, while denser water settles at the bottom.

Explanation: This density difference is exactly what makes separation using a separating funnel possible.

Question 2: Why does sand not sublime along with camphor during Activity 5.7?

Answer: Sand does not undergo sublimation because it lacks the property of changing directly from solid to vapour.

Explanation: Only camphor sublimes and rises as vapour, leaving sand behind in the china dish — achieving clean separation.

Question 3: Why is filtration sometimes not enough to clear muddy water completely?

Answer: Filtration removes larger particles only. The water often remains cloudy because very fine particles pass through the filter paper.

Explanation: This is why additional techniques like centrifugation or coagulation are needed for complete purification.

Question 4: How does centrifugation separate components of blood?

Answer: When blood is spun at high speed, the centrifugal force pushes heavier components (like red blood cells) outward and downward, while lighter plasma stays on top.

Explanation: This is widely used in laboratories to separate blood into its components for medical testing.

Question 5: What is a paperfuge, and why is it useful?

Answer: A paperfuge is a hand-powered, low-cost device that performs centrifugation without electricity.

Explanation: It helps detect diseases like malaria and anaemia in remote areas lacking access to electric centrifuge machines.

Question 6: How does coagulation help in making paneer (cheese) from milk?

Answer: Adding acid (lemon juice or vinegar) to milk causes its proteins to coagulate, clumping together to form paneer.

Explanation: This is a real-life, everyday example of coagulation outside water purification.

Question 7: Why can’t physical methods separate the components of an alloy?

Answer: When metals are melted and mixed, they form a new homogeneous material at the atomic level — the components are no longer distinguishable as separate substances.

Explanation: This is why alloys like brass and bronze require chemical methods, not physical separation, to break down.

Question 8: Why is blood considered a colloid and not a suspension or solution?

Answer: Blood cells cannot be seen with the naked eye (ruling out suspension) but blood can be separated by centrifugation and it coagulates — properties of a colloid.


Explanation: True solutions don’t separate by centrifugation, and true suspensions have visible particles — blood fits neither category perfectly.

Question 9: What happens during sedimentation in the coagulation process?

Answer: After alum causes fine particles to clump together, these larger clumps settle down by gravity — this settling is called sedimentation.

Explanation: Once settled, the clear water above can be separated by decantation or filtration.

Question 10: Give two examples of heterogeneous mixtures involving a gas as one component.

Answer: Smoke (solid particles suspended in air) and fog (liquid water droplets suspended in air).

Explanation: Unlike most gas mixtures (which are homogeneous), these specific examples contain visibly distinct dispersed particles, making them heterogeneous.

3-Mark Questions

Question 1: Describe the steps to separate mustard oil and water using a separating funnel.

Answer

🧪 Separating Immiscible Liquids

• Reopen the stopcock to collect the remaining oil layer separately
• Pour the mustard oil-water mixture into a separating funnel; let it stand undisturbed
• Two layers form — oil on top (less dense), water at the bottom (denser)
• Open the stopcock slowly to drain the lower water layer into a container
• Close the stopcock when the water is almost fully drained
• Discard the small middle portion containing both liquids

Explanation: This method relies entirely on the density difference between the two immiscible liquids.

Question 2: Explain the setup and process of separating camphor from sand using sublimation.

Answer

🔬 Sublimation Setup

1. Sand remains in the china dish since it does not sublime
2. Place the camphor-sand mixture in a clean, dry china dish on a wire gauze and tripod stand
3. Invert a cotton-plugged glass funnel over the china dish
4. Heat gently using a burner placed under the wire gauze
5. Camphor sublimes, rising as vapour and depositing as white solid crystals on the inner wall of the funnel

Explanation: The cotton plug prevents vapour from escaping, allowing camphor to deposit back as a solid inside the funnel.

Question 3: Why is filtration not always sufficient to clean muddy water? Suggest two alternative techniques and explain why they work.

Answer: Filtration only removes large particles; very fine suspended particles pass through filter paper, leaving water cloudy.

Two Alternative Techniques:

Question 4: Compare solutions, suspensions, and colloids based on particle size and settling behaviour, with one example each.

Answer

Explanation: Particle size is the deciding factor — the smaller the particle, the more stable and uniform the mixture remains over time.

Question 5: Describe how centrifugation is used to separate components of blood, and name the components obtained.

Answer: Blood is placed in a test tube and spun in a centrifuge machine at high speed.

Components Obtained (top to bottom):

Plasma (lightest — stays on top)
Platelets and white blood cells (middle layer)
Red blood cells (heaviest — settles at bottom)

Explanation: This separation is crucial for blood donation — components are stored separately in blood banks and used as needed for transfusions.

Question 6: Explain the process of coagulation in purifying muddy water, including the role of alum.

Answer

💧 Coagulation Process

1. The settled impurities are separated from clear water by decantation or filtration
2. Powdered alum is added to a beaker of muddy water
3. Alum acts as a coagulant, causing fine suspended impurities to clump together
4. These larger clumps settle by gravity (sedimentation)

Explanation: Alum specifically targets particles too fine for ordinary filtration, making it essential in municipal water treatment.

Question 7: What would happen if two immiscible liquids of the same density were mixed in a separating funnel?

Answer: If both liquids have the same density, no clear separation into distinct layers would occur, since neither liquid would consistently float or sink.

Explanation: The separating funnel method depends entirely on density differences — without this difference, the layers may mix unpredictably or form an unstable interface.

Question 8: How are emulsions different from regular colloids? Give two examples.

Answer: Emulsions are a special type of colloid where both the dispersed phase and dispersion medium are liquids.

Two Examples:

1. Butter — water-in-oil emulsion
2. Milk — oil-in-water emulsion

Explanation:
Regular colloids can have other states (solid-in-liquid, gas-in-liquid), but emulsions are specifically liquid-liquid colloidal mixtures, stabilised by emulsifying agents like proteins.

Question 9: Why do most gas mixtures tend to be homogeneous, while smoke and fog are heterogeneous?

Answer: Gas particles move freely in all directions, mixing easily and uniformly — making most gas mixtures homogeneous (like hydrogen-oxygen rocket fuel).

Smoke and fog are exceptions because they contain solid particles or liquid droplets dispersed in air — these don’t mix at the molecular level like gases do.

Explanation: The presence of a non-gaseous dispersed phase (solid in smoke, liquid in fog) makes these mixtures heterogeneous despite air being involved.

Question 10: Explain why proteins in milk are called emulsifying agents.

Answer: Proteins in milk act as emulsifying agents because they stabilise the emulsion, preventing the oil and water components from separating into distinct layers.

Explanation: Without these proteins, the fat droplets (oil) in milk would separate from the water, just like oil and water naturally do when left undisturbed.

5-Mark Questions


Question 1: Describe Activity 5.6 in detail — the separation of mustard oil and water. Explain the principle involved and one real-world application of this technique.

Answer🧪 Separating Immiscible Liquids — Full Process

Principle: Mustard oil and water are immiscible liquids with different densities. The lighter liquid (oil) floats above the denser liquid (water), allowing separation using a separating funnel.

Step-by-Step Activity 5.6

Why This Works

The density difference between oil and water means they never mix at the molecular level — they always form two distinct, separable layers.

Real-World Application

This same principle is used in industrial oil-water separation — for example, treating oily wastewater from factories, or separating crude oil from water during extraction processes.

Brain Anchor: 🔥 Lighter floats, heavier sinks — gravity does the sorting, the funnel does the collecting.

Question 2: Compare suspensions and colloids in detail on the basis of particle size, visibility, settling, filtration, and Tyndall effect. Give two examples of each.

Answer: 🔬 Suspensions vs Colloids — Complete Comparison

Comparison Table

Key Differences Explained

Suspensions have larger, heavier particles that gravity eventually pulls down. This is why muddy water, left undisturbed, develops a clear layer on top with mud settled at the bottom.

Colloids have medium-sized particles — large enough to scatter light (showing the Tyndall effect) but small enough to stay dispersed indefinitely, never settling.

Why This Matters Practically

Brain Anchor: 🔥 Suspension gives up and settles. Colloid never gives up — it just floats forever.

Question 3: Explain centrifugation and coagulation as methods to purify muddy water. Include their working principles and one real-world application for each.

Answer: 💧 Two Methods to Purify Muddy Water

A. Centrifugation

Principle: Spinning a mixture at high speed creates a centrifugal force that pushes heavier particles outward, while lighter liquid stays near the centre/top.

Process:

1. Mixture is placed in a centrifuge tube
2. Machine spins the tube at high speed
3. Tubes become horizontal; heavier particles move outward and settle at the bottom
4. Lighter liquid remains on top

Real-World Application: Used in laboratories to separate blood components (red blood cells from plasma) and in chemical industries for various separations.

B. Coagulation

Principle: Adding a coagulant (like alum) causes fine suspended particles to clump together into larger masses that can settle by gravity.

Process:

1. Powdered alum is added to muddy water
2. Alum causes particles to clump (coagulate)
3. Larger clumps settle (sedimentation)
4. Clear water is separated by decantation or filtration

Real-World Application: Used in municipal water treatment plants to purify drinking water, and in making paneer from milk using acid as a coagulant.

Quick Comparison

Brain Anchor: 🔥 Centrifugation spins them apart. Coagulation sticks them together.

Question 4: What are colloids? Explain their properties, classify emulsions, and explain how colloids differ from both solutions and suspensions.

Answer: 🧪 Understanding Colloids

Definition: A colloid is a mixture where dispersed particles range from 1 to 1000 nm — too small to settle, but large enough to scatter light.

Key Properties of Colloids

Components of a Colloid

Classification of Emulsions

Emulsions are colloids where both dispersed phase and dispersion medium are liquids.

Emulsifying agents (like proteins in milk) stabilise these emulsions, preventing separation.

Colloid vs Solution vs Suspension

Brain Anchor: 🔥 Colloid is the in-between mixture — too small to settle, too big to disappear.

Question 5: A mixture of sand, common salt, and naphthalene is given. Describe a complete separation scheme using sublimation, filtration, and any other method needed. Justify each step with reference to the properties of heterogeneous mixture separation.

Answer : Complete Separation Scheme

Given: Sand (insoluble, non-sublimable) + Common salt (soluble) + Naphthalene (sublimable)

Step-by-Step Separation

Justification for Each Step

Step 1 (Sublimation): Naphthalene is the only sublimable component here — it transitions directly from solid to vapour, leaving sand and salt completely undisturbed. This is a property unique to certain solids covered under heterogeneous mixture separation.

Step 2 & 3 (Dissolving + Filtration): Since sand is insoluble in water, while salt dissolves completely, this difference allows clean separation using filtration — a method that exploits solubility differences within a heterogeneous-turned-homogeneous system.

Step 4 (Evaporation): Removing water from the salt solution recovers pure crystalline salt.

Why This Multi-Step Approach is Necessary

No single technique from Section 5.4 could separate all three components — sublimation handles naphthalene, but sand and salt require a different property (solubility) to be separated from each other.

Brain Anchor: 🔥 Three different solids, three different escape routes — vapour, dissolve, or stay solid.

MCQs (Multiple Choice Questions)

Question 1: Which apparatus is used to separate two immiscible liquids?

(a) Filter paper
(b) Separating funnel
(c) China dish
(d) Centrifuge

Answer: (b) Separating funnel

Explanation: It uses density differences to drain the denser liquid first through a stopcock, leaving the lighter liquid behind.

Question 2: During sublimation, camphor changes from:

(a) Liquid to gas
(b) Solid to liquid
(c) Solid directly to vapour
(d) Gas to solid

Answer: (c) Solid directly to vapour

Explanation: This happens below camphor’s melting point, skipping the liquid state entirely.

Question 3: Which of the following is NOT an example of a suspension?

(a) Sand in water
(b) Sawdust in water
(c) Tea leaves in water
(d) Salt in water

Answer: (d) Salt in water

Explanation: Salt dissolves completely, forming a true solution, not a suspension where particles remain visible and settle.

Question 4: Centrifugation works on the principle of:

(a) Density difference using centrifugal force
(b) Boiling point difference
(c) Solubility difference
(d) Sublimation

Answer: (a) Density difference using centrifugal force

Explanation: Heavier particles are pushed outward during high-speed spinning, separating them from lighter liquid.

Question 5: Which substance is commonly used as a coagulant in water purification?

(a) Sulfuric acid
(b) Alum (fitkari)
(c) Copper sulfate
(d) Camphor

Answer: (b) Alum (fitkari)

Explanation: Alum causes fine suspended particles in muddy water to clump together and settle.

Question 6: Blood is classified as a:

(a) True solution
(b) True suspension
(c) Colloid
(d) Pure substance

Answer: (c) Colloid

Explanation: Blood cells cannot be seen with the naked eye (ruling out suspension), yet blood separates by centrifugation and coagulates — properties unlike a true solution.

Question 7: Milk is an example of which type of emulsion?

(a) Water-in-oil
(b) Oil-in-water
(c) Gas-in-liquid
(d) Solid-in-liquid

Answer: (b) Oil-in-water

Explanation: Milk’s dispersed phase is oil/fat droplets, while the dispersion medium is water — making it an oil-in-water emulsion.

Question 8: Which of the following correctly orders blood components from top to bottom after centrifugation?

(a) Red blood cells, plasma, platelets
(b) Plasma, platelets/white blood cells, red blood cells
(c) Platelets, red blood cells, plasma
(d) Red blood cells, platelets, plasma

Answer: (b) Plasma, platelets/white blood cells, red blood cells

Explanation: Plasma is lightest and stays on top; red blood cells are heaviest and settle at the bottom.

Question 9: A low-cost, hand-powered device that performs centrifugation without electricity is called a:

(a) Separating funnel
(b) China dish
(c) Paperfuge
(d) Condenser

Answer: (c) Paperfuge

Explanation: It helps detect diseases like malaria and anaemia in remote areas lacking electricity.

Question 10: Which of these CANNOT be separated by physical methods?

(a) Sand and salt
(b) Oil and water
(c) Components of an alloy like brass
(d) Camphor and sand

Answer: (c) Components of an alloy like brass

Explanation: Alloys form a new homogeneous material at the atomic level when metals are melted together — physical methods cannot separate them.

Assertion–Reason Questions

Instructions: Choose the correct option —

(i) Both A and R are true, and R is the correct explanation of A
(ii) Both A and R are true, but R is NOT the correct explanation of A
(ii) A is true, but R is false
(iv) A is false, but R is true

Question 1:
Assertion (A): Mustard oil forms the upper layer when mixed with water.
Reason (R): Mustard oil has a lower density than water.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Density difference is exactly why lighter mustard oil floats above denser water in a separating funnel.

Question 2:
Assertion (A): Sand can be separated from camphor using sublimation.
Reason (R): Both sand and camphor sublime when heated.

Answer: (iii) A is true, but R is false.

Explanation: A is correct — sublimation does separate them. But R is false — only camphor sublimes; sand does not undergo sublimation at all.

Question 3:
Assertion (A): Filtration alone is sufficient to completely clear muddy water.
Reason (R): Filter paper can trap even the finest suspended particles.

Answer: (iv) A is false, and R is false

Explanation: A is false — filtration is not always sufficient; water often remains cloudy because fine particles pass through filter paper.

Question 4:
Assertion (A): Centrifugation separates blood into plasma and blood cells.
Reason (R): Centrifugal force pushes heavier particles outward and downward during high-speed spinning.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: This is precisely how centrifuges separate denser red blood cells from lighter plasma in laboratory blood testing.

Question 5:
Assertion (A): Alum is used as a coagulant in water purification.
Reason (R): Alum causes fine suspended particles to clump together and settle.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: This clumping action (coagulation) is exactly why alum helps clear muddy water by forming larger, settleable particles.

Question 6:
Assertion (A): Blood is a true suspension.
Reason (R): Blood components can be separated by centrifugation.

Answer: (iv) A is false, but R is true.

Explanation: A is false — blood is a colloid, not a true suspension, since its particles are not visible to the naked eye. R is a true fact about blood but doesn’t make A correct

Question 7:
Assertion (A): The components of an alloy like bronze can be separated using a separating funnel.
Reason (R): Alloys are homogeneous mixtures formed at the atomic level when molten metals combine.

Answer: (iv) A is false, but R is true.

Explanation: A is false — a separating funnel works only for liquid-liquid immiscible mixtures, not solid alloys. R correctly explains why alloys cannot be separated by any physical method

Question 8:
Assertion (A): Milk and butter are both examples of the same type of emulsion.
Reason (R): Milk is an oil-in-water emulsion, while butter is a water-in-oil emulsion.

Answer: (iv) A is false, but R is true.

Explanation: A is false — milk and butter are different types of emulsions. R correctly explains this difference.

Question 9:
Assertion (A): Smoke is a heterogeneous mixture.
Reason (R): Smoke contains solid particles suspended in air.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: The presence of visible solid particles dispersed in air is exactly why smoke does not mix uniformly like typical gas mixtures.

Question 10:
Assertion (A): A paperfuge can replace an electric centrifuge in remote areas.
Reason (R): It uses hand-powered spinning to separate heavier components from lighter ones, just like a lab centrifuge.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: This low-cost device works on the same centrifugal principle, making it a practical alternative where electricity is unavailable.

Fill in the Blanks

Question 1: Liquids that do not mix and form separate layers are called __________ liquids.

Answer: immiscible

Explanation: Mustard oil and water are a common example of immiscible liquids.

Question 2: The apparatus used to separate two immiscible liquids is called a __________.

Answer: separating funnel

Explanation: It works by exploiting the density difference between the two liquids.

Question 3: The change of vapour directly into solid without becoming a liquid is called __________.

Answer: deposition

Explanation: This is the reverse of sublimation, seen when camphor vapour solidifies on a cool surface.

Question 4: Heterogeneous mixtures where solid particles remain suspended and visible are called __________.

Answer: suspensions

Explanation: Sand in water and tea leaves in water are common examples

Question 5: The process of spinning a mixture at high speed to separate components based on density is called __________.

Answer: centrifugation

Explanation: It is widely used to separate blood into plasma and blood cells.

Question 6: __________ is a chemical substance added to muddy water to make fine particles clump together.

Answer: Alum (fitkari)

Explanation: Alum acts as a coagulant, causing impurities to settle through coagulation.

Question 7: The settling of clumped particles by gravity is called __________.

Answer: sedimentation

Explanation: This occurs after coagulation, when larger clumps sink to the bottom.

Question 8: Colloids in which both the dispersed phase and dispersion medium are liquids are called __________.

Answer: emulsions

Explanation: Milk and butter are common examples of emulsions.

Question 9: A homogeneous mixture of two or more metals is called an __________.

Answer: alloy

Explanation: Brass, bronze, and stainless steel are common examples that cannot be separated physically.

Question 10: Proteins in milk act as __________ agents, stabilising the emulsion.

Answer: emulsifying

Explanation: These agents prevent the oil and water components of milk from separating into layers.

True or False

Question 1: Mustard oil and water form a homogeneous mixture.

Answer: ❌ False

Correction: Mustard oil and water form a heterogeneous mixture since they are immiscible and form separate layers.

Question 2: Sand undergoes sublimation along with camphor when heated.

Answer: ❌ False

Correction: Only camphor sublimes; sand does not undergo sublimation, which is why they can be separated.

Source: Page 84, Section 5.4.2, Paragraph after Activity 5.7

Question 3: Filtration is always sufficient to completely clear muddy water.

Answer: ❌ False

Correction: Filtration removes only larger particles; fine particles often pass through, leaving water cloudy. Centrifugation or coagulation may be needed.

Source: Page 85, Section 5.4.3, Paragraph 1

Question 4: During centrifugation, heavier particles move outward and settle at the bottom of the tube.

Answer: ✅ True

Explanation: Centrifugal force pushes denser particles outward, while lighter liquid remains near the top.

Question 5: Alum acts as a coagulant in the purification of muddy water.

Answer: ✅ True

Explanation: Alum causes fine suspended particles to clump together, enabling easier removal by sedimentation or filtration.

Question 6: Blood is an example of a true suspension.

Answer: ❌ False

Correction: Blood is a colloid, not a true suspension — its particles are not visible to the naked eye, unlike typical suspensions.

Question 7: Milk is an example of a water-in-oil emulsion.

Answer: ❌ False

Correction: Milk is an oil-in-water emulsion, not water-in-oil. Butter and body lotions are examples of water-in-oil emulsions.

Question 8: The components of an alloy can be separated using physical methods like filtration.

Answer: ❌ False

Correction: Alloys cannot be separated by physical methods since they form a new homogeneous material at the atomic level.

Question 9: A paperfuge requires electricity to function.

Answer: ❌ False

Correction: A paperfuge is hand-powered and works without electricity, making it useful in remote areas.

Question 10: Smoke and fog are examples of heterogeneous mixtures involving gas.

Answer: ✅ True

Explanation: Smoke contains solid particles and fog contains liquid droplets, both dispersed in air, making them heterogeneous despite air being a gas.

5.5 Tyndall Effect

Chapter 5 | Exploring Mixtures and Their Separation | Grade 9

1-Mark Questions

Question 1: What is the Tyndall effect?

Answer: The Tyndall effect is the scattering of light by particles in a colloid or suspension.

Explanation: This scattering makes the path of a light beam visible as it passes through the mixture.

Question 2: Who first explained the scattering of light by particles?

Answer: John Tyndall, a scientist who studied light scattering by particles.

Explanation: The effect is named after him in recognition of his pioneering work

Question 3: Does the Tyndall effect occur in a true solution?

Answer: No, the Tyndall effect does not occur in a true solution.

Explanation: Solution particles are too small (less than 1 nm) to scatter light.

Question 4: Name the component of a colloid in which the dispersed phase is suspended.

Answer: The dispersion medium.

Explanation: It is the component that surrounds and holds the dispersed particles.

Question 5: Give one everyday example where the Tyndall effect is observed.

Answer: Floodlights in a sports stadium show the Tyndall effect.

Explanation: Dust and moisture particles in the air scatter the light, making the beams visible.

Question 6: What is the dispersed phase in a colloid?

Answer: The dispersed phase is the solute-like component or the dispersed particles in a colloid.

Explanation: It is the substance that is spread throughout the dispersion medium.

Question 7: Name one mixture that shows the Tyndall effect, based on Activity 5.1.

Answer: Chalk powder and water (or milk and water) shows the Tyndall effect.

Explanation: Both are heterogeneous-type mixtures (suspension and colloid) whose particles scatter light.

Question 8: What type of mixture is needed to observe scattering of light?

Answer: A colloid or a suspension is needed to observe light scattering.

Question 9: What causes the visible beam of light entering a dark room through a small hole?

Answer: Dust and smoke particles in the air scatter the light, making the beam visible.

Explanation: This is a natural, everyday demonstration of the Tyndall effect.

Question 10: Is the Tyndall effect seen in salt water?

Answer: No, salt water does not show the Tyndall effect.

Explanation: Salt water is a true solution with particles too small to scatter light.

2-Mark Questions

Question 1: Why was the light beam path visible in mixture C (milk and water) in Activity 5.1, even though the mixture appeared homogeneous?

Answer: Mixture C is a colloid — though it looks uniform, its dispersed particles scatter light, making the beam’s path visible.

Explanation: This proves that appearance alone cannot confirm whether a mixture is a true solution or a colloid.

Question 2: Define dispersed phase and dispersion medium with one example.

Answer:

Explanation: Together, these two components make up a complete colloidal system.

Question 3: Why doesn’t a true solution show the Tyndall effect?

Answer: A true solution’s particles are smaller than 1 nm — far too tiny to scatter light.

Explanation: Light simply passes straight through without any visible deflection or scattering.

Question 4: Explain why floodlights in a stadium often show visible beams on a foggy or dusty night.

Answer: Tiny dust or fog particles in the air act as scattering centres, causing light to spread and become visible as distinct beams.

Explanation: This is a large-scale, real-world demonstration of the Tyndall effect happening in air rather than a liquid.

Question 5: In Activity 5.1, how do the laser beam observations help classify the three mixtures (salt, chalk, milk)?

Answer:

Explanation: The presence or absence of scattered light directly indicates particle size and mixture classification.

3-Mark Questions

Question 1: Explain why scattering of light occurs in a colloid or suspension but not in a true solution.

Answer: Scattering depends entirely on particle size.

• Solutions — particles smaller than 1 nm — too tiny to interact with light waves, so light passes straight through
• Colloids — particles between 1–1000 nm — large enough to scatter light in different directions
• Suspensions — particles larger than 1000 nm — also scatter light effectively

Explanation: This is why the Tyndall effect serves as a simple visual test to distinguish a true solution from a colloid or suspension.

Question 2: Describe two real-life situations (other than the laboratory) where the Tyndall effect can be observed.

Answer:

1. Sunlight through tree leaves — bright rays appear visible when sunlight passes through small gaps between leaves of a dense tree, scattered by dust and moisture particles in the air
2. Floodlights in a sports stadium — visible light beams form due to scattering by dust and fog particles, especially on cooler or humid nights

Explanation: Both examples involve air acting as a colloid-like medium, with suspended particles scattering light to create visible beams

Question 3: What are the components of a colloid? Explain dispersed phase and dispersion medium using milk as an example.

Answer:

Components of a Colloid

Explanation: Milk is classified as an oil-in-water emulsion, a special type of colloid where both phases are liquids — the fat droplets (dispersed phase) remain suspended in water (dispersion medium) without settling.

Question 4: A student claims that since milk looks uniform like sugar solution, both must be true solutions. Use the Tyndall effect to correct this misconception.

Answer: This claim is incorrect. While milk looks homogeneous, passing a laser beam through it reveals the truth:

• Sugar solution — beam path is invisible (particles too small to scatter light) → true solution
• Milk — beam path is visible (particles scatter light) → colloid, not a solution

Explanation: Visual appearance can be misleading; the Tyndall effect test reliably distinguishes a true solution from a colloid, since colloidal particles (1–1000 nm) are large enough to scatter light even though they cannot be seen individually.

Question 5: Why is the Tyndall effect considered an important practical test in classifying mixtures?

Answer: The Tyndall effect is important because it allows quick visual classification of mixtures without needing complex lab equipment.

Key Reasons:

1. It distinguishes solutions from colloids/suspensions instantly using just a light source
2. It works even when a mixture appears uniform to the naked eye (like milk)
3. It requires minimal equipment — just a laser pointer or flashlight

Explanation: This makes it a practical, accessible tool for students and scientists alike to verify mixture type before using more advanced separation techniques.

5-Mark Question

Question 1: Explain the Tyndall effect in detail. Describe how it was demonstrated in Activity 5.1, explain the role of particle size, define dispersed phase and dispersion medium, and give two real-life examples of this phenomenon.

Answer

🔬 The Tyndall Effect — Complete Explanation

Definition: The Tyndall effect is the scattering of light by particles present in a colloid or suspension, making the path of a light beam visible. It is named after the scientist John Tyndall, who first explained this phenomenon.

How Activity 5.1 Demonstrated It

A laser beam was passed through three different mixtures, and observations were recorded from the side, perpendicular to the beam:

This experiment proved that even though milk appeared homogeneous like the salt solution, it actually scattered light — revealing it as a colloid, not a true solution.

Role of Particle Size

Scattering depends entirely on how large the particles are:

Key Insight: Only particles large enough (colloid or suspension range) can interact with and scatter light waves; solution particles are simply too small.

Dispersed Phase and Dispersion Medium

Every colloid has two components:

Example: In milk, fat droplets form the dispersed phase, while water acts as the dispersion medium — together making milk an oil-in-water emulsion.

Two Real-Life Examples

1. Sunlight through tree leaves — bright rays become visible when passing through small gaps in dense foliage, scattered by dust and moisture particles in the air
2. Floodlights in a sports stadium — visible light beams form at night due to scattering by dust, fog, or moisture particles suspended in the air

Why This Matters

The Tyndall effect is a simple, reliable visual test that helps classify unknown mixtures as solutions, colloids, or suspensions — all without needing complicated lab equipment.

Brain Anchor: 🔥 No scatter, it’s a solution. Some scatter, it’s a colloid or suspension — the beam never lies.

Assertion–Reason Questions

Instructions: Choose the correct option —

• (i) Both A and R are true, and R is the correct explanation of A
• (ii) Both A and R are true, but R is NOT the correct explanation of A
• (iii) A is true, but R is false
• (iv) A is false, but R is true

Question 1:
Assertion (A): The Tyndall effect is observed when light passes through a colloid.
Reason (R): Colloidal particles are large enough to scatter light.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Colloid particles range from 1–1000 nm — exactly the size needed to scatter light and make the beam’s path visible.

Question 2:
Assertion (A): A true solution shows the Tyndall effect strongly.
Reason (R): Solution particles are smaller than 1 nm in size.

Answer: (iv) A is false, but R is true.

Explanation: A is false — a true solution does NOT show the Tyndall effect at all. R is correct on its own, but since the particles are too small to scatter light, it actually explains the opposite of A (the absence of the effect, not its presence).

Question 3:
Assertion (A): Floodlights in a sports stadium often show visible light beams at night.
Reason (R): Dust and moisture particles in the air scatter the light, demonstrating the Tyndall effect.

Answer: (i) Both A and R are true, and R is the correct explanation of A.

Explanation: Air containing suspended dust or fog particles behaves like a colloid — scattering light exactly as described by the Tyndall effect.

Fill in the Blanks

Question 1: The scattering of light by particles in a colloid or suspension is known as the ______ effect.

Answer: Tyndall

Explanation: This effect makes the path of a light beam visible, and is named after scientist John Tyndall, who first explained it.

Question 2: In a colloid, the solute-like particles spread throughout the mixture are called the ______ phase.

Answer: dispersed

Explanation: The dispersed phase is suspended within the dispersion medium — together they make up the complete colloidal system.

Question 3: A true solution does not show the Tyndall effect because its particles are smaller than ______ nm.

Answer: 1

Explanation: Particles this small are too tiny to interact with and scatter light waves, so the beam passes straight through undetected.

MCQs (Multiple Choice Questions)

Question 1: Who first explained the scattering of light by particles?

(a) John Dalton
(b) John Tyndall
(c) Isaac Newton
(d) Albert Einstein

Answer: (b) John Tyndall

Explanation: The effect is named after this scientist, who studied how particles scatter light in colloids and suspensions.

Question 2: Which of the following mixtures will NOT show the Tyndall effect?

(a) Milk and water
(b) Salt and water
(c) Chalk powder and water
(d) Smoke in air

Answer: (b) Salt and water

Explanation: Salt water is a true solution with particles smaller than 1 nm — too small to scatter light. The other three are colloids or suspensions, which do scatter light.

Question 3: In a colloid, the component in which the dispersed phase is suspended is called the:

(a) Dispersed phase
(b) Solute
(c) Dispersion medium
(d) Precipitate

Answer: (c) Dispersion medium

Explanation: Together, the dispersed phase (particles) and the dispersion medium (surrounding substance) make up a complete colloid.

True or False

Question 1: The Tyndall effect occurs only in suspensions, not in colloids.

Answer: ❌ False

Correction: The Tyndall effect occurs in both colloids and suspensions — both have particles large enough to scatter light. Only true solutions fail to show this effect.

Question 2: Sunlight passing through small gaps between tree leaves can demonstrate the Tyndall effect.

Answer: ✅ True

Explanation: Dust and moisture particles in the air scatter the sunlight, making the rays visible — a natural, everyday example of this effect.

Question 3: A mixture’s appearance alone is enough to confirm whether it is a true solution or a colloid.

Answer: ❌ False

Correction: Appearance can be misleading — milk looks homogeneous like a solution, but the Tyndall effect test reveals it is actually a colloid since it scatters light, unlike a true solution.