Chapter 9 of the class 8 NCERT textbook, Curiosity, is all about solute, solvent, and how they combine to form solvents. The chapter explains in detail everything related to solutes, solvents, and solutions, but you need to keep in mind only the most important points for the exams.
For that, you need short notes for quick revision and memorisation. And “The Amazing World of Solutes, Solvents, and Solutions Notes Class 8 Chapter 9” Science is what we offer to you.
We have also created a PDF of short notes; the link is at the end of the page.
Happy Learning!
What Are Solute, Solvent, and Solution?
Solution
A uniform mixture is formed when a solid is mixed with a liquid (or when two liquids are mixed, or when gases mix).
Solute
The component that dissolves in the solvent. In solid-liquid mixtures, it’s the solid. In liquid-liquid or gas mixtures, it’s the substance present in smaller amounts.
Solvent
The component that dissolves the solute. In solid-liquid mixtures, it’s the liquid. In mixtures of equal phases, it’s the substance present in the larger amount.
Formation Formula
Solute + Solvent = Solution
Identifying Solute vs Solvent
| Substance in the largest amount | Solute | Solvent |
|---|---|---|
| Solid + Liquid | The solid component | The liquid component |
| Liquid + Liquid | Substance in a smaller amount | Substance in larger amount |
| Gas + Gas | Substance in smaller amount | Substance in largest amount |
Air is a gaseous solution.
Solvent: Nitrogen
(present in the largest amount)
Solutes: Oxygen, Argon,
Carbon dioxide and other gases
(present in smaller amounts)
How Much Solute Can a Fixed Amount of Solvent Dissolve?
Activity 9.1 Salt Dissolution
Procedure:
▸Fill a glass tumbler half
▸Add salt spoon by spoon and stir
Note when it stops dissolving
Observation:
▸Salt dissolves at first
▸After some time, extra salt remains undissolved at the bottom
Click on saturated and unsaturated
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Concentration
The amount of solute present in a fixed quantity of solution (or solvent).
| Type | Definition |
|---|---|
| Dilute Solution | Less amount of solute in a fixed quantity of solution |
| Concentrated Solution | More amount of solute in a fixed quantity of solution |
Solubility
The maximum amount of solute that dissolves in a fixed quantity of the solvent
(at a particular temperature).
How does temperature affect the solubility of a solute?
Activity 9.2
Procedure:
▸Take 50 mL of water at 20°C
▸Add baking soda till some remains undissolved
▸Heat to 50°C → it dissolves
▸Add more baking soda
▸Heat to 70°C → more dissolves
Observation:
▸Solubility increases with temperature
Key Finding:
| Temperature | Amount of Baking Soda Dissolved |
|---|---|
| 20°C | Least |
| 50°C | More |
| 70°C | Most |
For most substances:
- Solubility increases as temperature increases
- A saturated solution becomes unsaturated on heating
Solubility of Gases
Gas Dissolution in Water
▸Oxygen and other gases dissolve in water in small amounts
▸Dissolved oxygen is essential for aquatic life
▸eg, plants, fish, and other organisms.
The mixture of gases in water is uniform because:
▸Gases dissolve evenly in water
▸Gases form a solution with water
Effect of Temperature on Gas Solubility
Solubility of gases generally decreases as temperature increases
| Temperature | Gas Solubility | Effect on Aquatic Life |
|---|---|---|
| Cold water | Higher solubility | Sufficient oxygen available for aquatic organisms |
| Warm/Hot water | Sufficient oxygen is available for aquatic organisms | Less oxygen dissolved; harmful for aquatic life |
Why Do Objects Float or Sink in Water?
Floating vs Sinking
▸Rice sinks; rice husk floats
▸Oil floats on water
- Floating and sinking depend on density
- Objects less dense than water float; more dense sink
What Is Density?
How do scientists define density?
Density
The mass present in a unit volume of a substance.
Formula:
Imp | Points:
▸Independent of shape and size
▸Depends on temperature and pressure
▸Pressure affects gases more; little effect on solids and liquids
Units of Density
| System | Unit | Abbreviation | Usage |
|---|---|---|---|
| SI Units | Kilogram per cubic metre | kg/m³ | Universal standard |
| For Liquids | Gram per millilitre | g/mL | Convenient for liquids |
| For Liquids | Gram per cubic centimetre | g/cm³ | Convenient for liquids |
Conversion factor for density
Conversion:
1 kg/m³ = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³
Density of Water (Reference Standard)
- Mass of 1 mL of water ≈ 1 g (at room temperature)
- 10 mL of water ≈ 10 g
- 100 mL of water ≈ 100 g
Relative Density
The ratio of the density of a substance to the density of water at the same temperature.
Formula
Relative density is
▸A number without units
▸Used to compare how dense a substance is compared to water
Example:
▸Relative density of aluminium = 2.7
▸This means aluminium is 2.7 times denser than water
Determination of density
The density of a substance is determined by measuring its mass and volumes
How to measure mass?
Activity 9.3
▸Switch ON the digital weighing balance
▸Set it to zero
▸Place a watch glass → reset again
▸Put an object (stone) on it
▸Note the reading
Observation:
Mass of stone = 16.400 g
How to measure volume?
Volume
The space occupied by an object.
SI Unit:
Cubic metre (m³) — volume of a cube with each side = 1 metre
Units of Volume
| Unit | Abbreviation | Equivalent to | Used For |
|---|---|---|---|
| Decimetre cube | dm³ | – | Smaller objects |
| Centimetre cube | cm³ or cc | – | Smaller objects |
| Litre | L | 1 dm³ | Liquids |
| Millilitre | mL | 1 cm³ | Liquids |
Measuring Cylinder
A narrow transparent cylindrical container used to measure the volume of liquids.
Characteristics:
One side open, one side closed
Markings on the transparent body indicate volume
Activity 9.4:
Key Observations (Short):
- Maximum volume: Total capacity (e.g., 100 mL)
- Least count: Smallest measurable value
Cylinder Table
| Capacity | Least Count |
|---|---|
| 10–25 mL | 0.1 mL |
| 100 mL | 1 mL |
| 250 mL | 2 mL |
| 500 mL | 5 mL |
Example (100 mL Cylinder):
10 mL ÷ 10 divisions = 1 mL per division
Choosing the Right Measuring Cylinder
| Scenario | Best Choice | Reason |
|---|---|---|
| Measuring 70 mL of water | 100 mL or 250 mL cylinder | Can measure in one step with good accuracy |
| Using 50 mL cylinder for 70 mL | Not suitable | Using a 500 mL cylinder |
| Using 500 mL cylinder | Less accurate | The smallest measurable volume is 5 mL (lower accuracy) |
Best Overall Choice:
100 mL measuring cylinder — provides good accuracy and convenience for most measurements.
Activity 9.5:
Procedure:
▸Place the measuring cylinder on a flat surface
▸Pour water to the 50 mL mark
▸Adjust using the dropper if needed
▸Observe the meniscus (curved surface)
▸Read at the bottom of the meniscus at eye level
▸Transfer water to the container
Meniscus
The curved surface formed by a liquid in a measuring cylinder.
| Type of Liquid | Where to Read |
|---|---|
| Colourless liquids | Bottom of the meniscus |
| Coloured liquids | Top of the meniscus |
Determining the volume of solid objects with regular shapes
Regular shapes
Objects like notebooks, shoe boxes, dice, or cuboids.
Procedure:
- Collect objects with cuboid shapes
- Measure length (l), width (w), and height (h) using a scale
Formula:
Volume = l × w × h
Example:
▸Length of notebook = 25 cm
▸Width = 18 cm
▸Height = 2 cm
Volume = 25 × 18 × 2 = 900 cm³
Volume of Objects with Irregular Shapes
Irregular shapes
Objects like stones, keys, or any non-uniform objects.
Challenge:
Cannot measure length, width, and height directly.
Solution:
Use the water displacement method.
Activity 9.6
Determining the volume of objects with irregular shapes
Procedure:
▸Take initial water volume = A
▸Immerse object → water level rises
▸Note final volume = B
Result:
Volume of object = B − A
Key Point:
The volume of water displaced = volume of the object
(because the object displaces water equal to its own volume)
Activity 9.7
Let us calculate the density
Formula:
Example Calculation:
▸Mass of stone = 16.400 g
▸Volume of stone = 5 cm³
Density = 16.400 ÷ 5 = 3.28 g/cm³
Let us dig deeper!
Density of Earth’s Layers
Earth is composed of several layers, each with different densities:
| Layer | Position | Density Characteristic |
|---|---|---|
| Crust | Outermost layer | Lightest |
| Upper Mantle | Below crust | Density increases |
| Lower Mantle | Below upper mantle | Density increases |
| Outer Core | Below mantle | Density increases |
| Inner Core | Centre of Earth | Below the upper mantle |
Observation:
- As depth increases, both pressure and temperature rise significantly
- Materials become heavier and more compact
- Density increases towards the centre of the Earth
Effect of temperature on density
General Rule:
- Density decreases with heating
- Density increases with cooling
Explanation:
As temperature increases:
- Particles of a substance move away and spread
- Volume increases
- Mass remains unchanged
Since Density = Mass / Volume:
- Volume increases → Density decreases
Why Hot Air Rises:
- Hot air is less dense than cool air around it
- Therefore, hot air moves upward
- A hot air balloon works on this principle.
Effect of pressure on density
The effect of Pressure differs by state of matter:
| State of Matter | Effect of Increased Pressure | Change in Volume | Change in Density | Explanation |
|---|---|---|---|---|
| Gases | Significant effect | Decreases | Increases | Particles move closer together |
| Liquids | Small effect | Minimal change | Minimal change | Liquids are nearly incompressible |
| Solids | Negligible effect | No significant change | Usually negligible | Particles are already very close together; hard to compress them further |
▶ Gases: Most affected by pressure — density changes significantly
▶ Liquids: Moderately affected — density changes slightly
▶ Solids: Least affected by pressure — density changes are negligible
Short Notes PDF for Chapter 9 Class 8 Science
FAQs: The Amazing World of Solutes, Solvents, and Solutions
What is the main difference between a solute and a solvent
The solute is the substance that gets dissolved (usually present in a smaller amount, like salt), while the solvent is the substance that does the dissolving (usually present in a larger amount, like water).
Why is air considered a solution?
Air is a gaseous solution because it is a uniform (homogeneous) mixture of gases. Nitrogen acts as the solvent (78%), while oxygen, carbon dioxide, and argon are the solutes.
What happens to a saturated solution when you heat it?
When a saturated solution is heated, it usually becomes unsaturated. This is because the solubility of most solids increases with temperature, allowing the solvent to dissolve more solute.
Why do aquatic animals prefer cold water over warm water?
The solubility of gases decreases as temperature increases. Cold water holds more dissolved oxygen than warm water, making it easier for fish and other aquatic organisms to breathe.
Why does an iron nail sink in water while a massive iron ship floats?
An iron nail is denser than water, so it sinks. However, a ship is built to be hollow, trapping a large volume of air. This reduces the ship’s average density to less than that of water, allowing it to float.
Does the shape of an object affect its density?
No, density is independent of shape and size. For example, a piece of clay has the same density whether it is shaped like a ball, a cube, or a thin sheet, because its mass-to-volume ratio remains constant.
What is the “Water Displacement Method” used for?
This method is used to find the volume of irregular objects (like a stone). By immersing the object in a measuring cylinder, the volume of water it displaces is exactly equal to the volume of the object itself.
What is Relative Density, and why does it have no units?
Relative Density is the ratio of a substance’s density to the density of water. Since it is a ratio of two similar quantities, the units cancel out, leaving a pure number
How does pressure affect the density of different states of matter?
Gases: Highly affected; increasing pressure significantly increases density.
Liquids: Minimally affected.
Solids: Negligible effect, as particles are already tightly packed and incompressible.
