CBSE Class 8 Science Revision Notes Chapter 9 The Amazing World of Solutes, Solvents, and Solutions

A solution is a uniform mixture in which one substance dissolves evenly in another substance.
Density, mass and volume help explain why some objects float while others sink in a liquid.

Many materials around us mix in different ways. Sugar dissolves evenly in water, while sand remains visible and settles at the bottom. These differences help us understand solutions and non-uniform mixtures.

These CBSE Class 8 Science Revision Notes Chapter 9 cover solutes, solvents, solubility, density and measurement. They follow the current 2026–27 textbook and include clear examples for revision.

Key Takeaways

• Solution: A uniform mixture in which the components are evenly distributed.
• Solubility: Maximum amount of solute that can dissolve in a fixed quantity of solvent at a given temperature.
• Density: Mass present per unit volume of a substance.
• Measurement: Mass is measured using a balance, while liquid volume is commonly measured using a measuring cylinder.

Revise Class 8 Science Chapter 9 in 30 Minutes

First 10 minutes: Mixtures, solute, solvent, solutions and concentration
Next 10 minutes: Solubility, temperature effects and dissolved gases
Final 10 minutes: Density, mass, volume, meniscus and displacement method

Need help practising solutions, density and measurement questions?
Access chapter-wise learning resources, interactive activities and doubt-solving support on the Extramarks Learning App. Sign Up Free

Mixtures in Class 8 Science Chapter 9 Notes

A mixture forms when two or more substances are combined physically.

Mixtures may be uniform or non-uniform.

Uniform Mixture

A uniform mixture has the same composition throughout.

Its components are evenly distributed and cannot be seen separately.

Examples include:

• Salt in water
• Sugar in water
• ORS
• Air
• Vinegar mixed with water

A uniform mixture is called a solution.

Non-Uniform Mixture

A non-uniform mixture does not have the same composition throughout.

Its components may be visible separately.

Examples include:

• Sand and water
• Chalk powder and water
• Sawdust and water
• Oil and water

In some non-uniform mixtures, heavier particles may settle, while lighter materials may float.

Uniform and Non-Uniform Mixtures

Solute, Solvent and Solution in CBSE Class 8 Science Chapter 9 Notes

A solution contains a solute and a solvent.

Solute

The substance that dissolves is called the solute.

Examples:

• Salt in salt water
• Sugar in sugar syrup
• Glucose in glucose solution

Solvent

The substance in which the solute dissolves is called the solvent.

Water is a common solvent because many substances dissolve in it.

Solution

The uniform mixture formed after the solute dissolves in the solvent is called a solution.

The relationship can be written as:

Solute + Solvent → Solution

Example

In salt water:

• Salt is the solute.
• Water is the solvent.
• Salt water is the solution.

Solid-Liquid Solution

When a solid dissolves in a liquid:

• Solid = solute
• Liquid = solvent

Example:

Sugar + water → sugar solution

Liquid-Liquid Solution

When two liquids form a solution, the liquid present in the smaller amount is generally called the solute.

The liquid present in the larger amount is called the solvent.

Special Case of Sugar Syrup

Sugar syrup may contain a large amount of sugar and a smaller amount of water.

Even then:

• Sugar remains the solute.
• Water remains the solvent.

This is because sugar dissolves in water.

Why Does Every Sip of ORS Taste the Same?

ORS contains sugar and salt dissolved in water.

The components are distributed evenly throughout the solution. Therefore, every sip contains a similar mixture of sugar, salt and water.

This is a key property of a uniform mixture.

Saturated and Unsaturated Solutions

A fixed amount of solvent cannot dissolve an unlimited amount of solute.

After a certain point, more solute remains undissolved.

Unsaturated Solution

An unsaturated solution can dissolve more solute at a given temperature.

For example, if one spoon of salt dissolves completely and more salt can still be added, the solution is unsaturated.

Saturated Solution

A saturated solution contains the maximum amount of solute that can dissolve at a particular temperature.

Any additional solute remains undissolved.

Comparison

Important Point

A solution that is saturated at one temperature may become unsaturated when the temperature is increased.

This happens because many solids dissolve more in warmer liquids.

Concentration of a Solution

Concentration refers to the amount of solute present in a fixed quantity of solution or solvent.

A solution may be dilute or concentrated.

Dilute Solution

A dilute solution contains a smaller amount of solute.

Example:

One spoon of salt in one glass of water.

Concentrated Solution

A concentrated solution contains a larger amount of solute in the same or a comparable amount of solution.

Example:

Four spoons of salt in one glass of water.

Dilute and Concentrated Are Relative Terms

A solution is called dilute or concentrated only in comparison with another solution.

For example, a solution with two spoons of salt may be more concentrated than one with one spoon. It may still be less concentrated than another solution containing four spoons.

Solubility in Class 8 Science Chapter 9 Notes

Solubility is the maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature.

It depends on:

• Nature of the solute
• Nature of the solvent
• Temperature
• In the case of gases, pressure may also affect solubility

Why Does Sugar Dissolve in Water but Not in Oil?

Different solvents dissolve different substances.

Water can dissolve sugar and salt well because of the nature of their particles and interactions. Oil has a different nature, so sugar and salt generally do not dissolve in it.

This shows that solubility depends on both the solute and the solvent.

Effect of Temperature on Solubility

Temperature affects the solubility of solids and gases differently.

Solubility of Solids in Liquids

For most solid substances, solubility increases when temperature increases.

For example:

• Less baking soda dissolves in water at 20°C.
• More dissolves at 50°C.
• Still more may dissolve at 70°C.

A saturated solution at a lower temperature may become unsaturated when heated.

Why Does Heating Help?

Heating allows the solvent to dissolve a larger amount of many solid solutes.

Therefore, undissolved solid may dissolve after heating.

Solubility of Gases in Liquids

The solubility of gases generally decreases when temperature increases.

Cold water can hold more dissolved oxygen than warm water.

Solubility of Solids and Gases

Dissolved Oxygen and Aquatic Life

Oxygen dissolves in water in small quantities.

Fish, aquatic plants and other organisms depend on this dissolved oxygen.

Cold water generally contains more dissolved oxygen than warm water.

If water becomes too warm:

• Less oxygen remains dissolved.
• Aquatic organisms may find it difficult to survive.
• The balance of the ecosystem may be affected.

Why Do Objects Float or Sink?

Some objects float in water, while others sink.

Examples:

• Sawdust may float.
• Sand usually sinks.
• Oil floats on water.
• A stone sinks.
• Ice floats.

Density helps explain many of these observations, though shape and other factors can also matter.

Density in Class 8 Science Revision Notes Chapter 9

Density describes how much mass is packed into a given volume.

Meaning of Density

Density is the mass present in unit volume of a substance.

The formula is:

Density = Mass ÷ Volume

or:

Density = Mass/Volume

Example

An aluminium block has:

• Mass = 27 g
• Volume = 10 cm³

Density:

27 ÷ 10 = 2.7 g/cm³

Therefore, the density of aluminium is 2.7 g/cm³.

Units of Density

The SI unit is:

kg/m³

Other commonly used units include:

• g/cm³
• g/mL

For liquids:

1 mL = 1 cm³

Density of Water

At room temperature:

• 1 mL of water has a mass close to 1 g.
• Density of water is close to 1 g/mL or 1 g/cm³.

Therefore:

• 10 mL water has a mass close to 10 g.
• 100 mL water has a mass close to 100 g.

Relative Density

Relative density compares the density of a substance with the density of water.

The formula is:

Relative density = Density of substance ÷ Density of water

Relative density has no unit because it is a ratio.

Example

Density of aluminium = 2.7 g/cm³
Density of water = 1 g/cm³

Relative density:

2.7 ÷ 1 = 2.7

Aluminium is therefore 2.7 times as dense as water.

Mass and Weight

Mass and weight are different scientific quantities.

Mass

Mass is the amount of matter present in an object.

Its units are:

• Gram
• Kilogram

Weight

Weight is the force with which Earth attracts an object.

It is measured in newtons.

Difference Between Mass and Weight

In daily language, people often use the words interchangeably. In science, they have different meanings.

Measuring Mass

A digital weighing balance may be used to measure mass.

Steps

1. Switch on the balance.
2. Check that it reads zero.
3. Place a clean watch glass or paper on the pan.
4. Press the tare or reset button.
5. Place the object.
6. Note the displayed reading.

The tare button removes the mass of the container from the final reading.

Measuring Volume in Class 8 Science Chapter 9 Notes

Volume is the space occupied by an object or substance.

Units of Volume

The SI unit is:

m³

Other units include:

• dm³
• cm³
• L
• mL

Important relationships:

1 L = 1 dm³

1 mL = 1 cm³

Measuring Liquid Volume

A measuring cylinder is commonly used to measure liquid volume.

It is:

• Narrow
• Transparent
• Marked with volume divisions
• Available in different capacities

Examples include:

• 10 mL
• 25 mL
• 50 mL
• 100 mL
• 250 mL

Smallest Division of a Measuring Cylinder

To calculate the smallest volume it can measure:

1. Find the difference between two numbered marks.
2. Count the small divisions between them.
3. Divide the difference by the number of divisions.

Example

Difference between 10 mL and 20 mL = 10 mL
Number of divisions = 10

Smallest division:

10 ÷ 10 = 1 mL

Choosing the Correct Measuring Cylinder

The selected cylinder should:

• Hold the required volume in one step.
• Have small divisions for better accuracy.

For measuring 70 mL, a 100 mL cylinder is usually more suitable than:

• A 50 mL cylinder, because it needs two measurements.
• A 500 mL cylinder, because its divisions may be too large.

Meniscus in a Measuring Cylinder

The curved surface of a liquid inside a measuring cylinder is called the meniscus.

Reading Colourless Liquids

For water and other colourless liquids:

• Read the bottom of the meniscus.
• Keep the eyes at the same level as the meniscus.

Reading Coloured Liquids

For coloured liquids, the top of the meniscus may be used as directed in the textbook setup.

Why Should the Eyes Be Level?

Looking from above or below can produce an incorrect reading.

This is called a reading error due to the viewing position.

Volume of Regular Solid Objects

For a cuboid:

Volume = Length × Width × Height

Example

A notebook has:

• Length = 25 cm
• Width = 18 cm
• Height = 2 cm

Volume:

25 × 18 × 2 = 900 cm³

Volume of Irregular Solid Objects

Irregular objects such as stones cannot be measured using a simple length formula.

Their volume can be found using water displacement.

Displacement Method

1. Pour water into a measuring cylinder.
2. Record the initial water level.
3. Lower the object fully into the water.
4. Record the final water level.
5. Subtract the initial level from the final level.

The formula is:

Volume of object = Final water volume − Initial water volume

Example

Initial water level = 50 mL
Final water level = 55 mL

Volume of stone:

55 − 50 = 5 mL

Since:

1 mL = 1 cm³

Volume of stone:

5 cm³

Calculating Density of an Irregular Object

Suppose:

• Mass of stone = 16.4 g
• Volume = 5 cm³

Density:

16.4 ÷ 5 = 3.28 g/cm³

Effect of Temperature on Density

Density is given by:

Density = Mass/Volume

When most substances are heated:

• Particles move farther apart.
• Volume increases.
• Mass remains the same.
• Density decreases.

Hot-Air Balloon

Hot air is less dense than cooler surrounding air.

Therefore, hot air rises and helps the balloon move upward.

Cooling

When many substances cool:

• Particles come closer.
• Volume decreases.
• Density increases.

Water behaves differently near its freezing point.

Effect of Pressure on Density

Pressure affects gases more than liquids and solids.

Gases

When pressure increases:

• Gas particles move closer.
• Volume decreases.
• Density increases.

Liquids

Liquids are nearly incompressible, so pressure causes only a small density change.

Solids

Particles in solids are already closely packed. Pressure usually has a negligible effect on their density.

Why Does Ice Float on Water?

Ice is less dense than liquid water.

When water freezes:

• Its particles arrange in a more open structure.
• The same mass occupies more volume.
• Density decreases.

Therefore, ice floats.

This is important for aquatic life because the floating ice layer leaves liquid water below it.

Density and Floating

In many simple cases:

• An object less dense than the liquid tends to float.
• An object denser than the liquid tends to sink.

Oil and Water

Oil floats because its density is lower than that of water.

Egg in Water

A raw egg usually sinks in plain water.

Adding salt increases the density of water. When the salt solution becomes dense enough, the egg may float.

Important Note

Density is a major factor in floating and sinking. The shape of an object and the amount of liquid displaced can also affect whether it floats.

Important Formulas in Class 8 Science Chapter 9

Solved Examples

Example 1: Density of a Sculpture

Mass = 225 g
Volume = 90 cm³

Density = 225/90

= 2.5 g/cm³

Since this is greater than the density of water, it is expected to sink.

Example 2: Density of an Object

Mass = 400 g
Volume = 40 cm³

Density = 400/40

= 10 g/cm³

Example 3: Find Volume

Mass of iron block = 600 g
Density = 7.9 g/cm³

Volume = Mass/Density

= 600/7.9

≈ 75.9 cm³

Example 4: Compare Two Objects

Object A:

• Mass = 200 g
• Volume = 40 cm³

Density:

200/40 = 5 g/cm³

Object B:

• Mass = 240 g
• Volume = 60 cm³

Density:

240/60 = 4 g/cm³

Object A is denser.

Common Mistakes in Class 8 Science Chapter 9

Confusing Solute and Solvent

The solute dissolves. The solvent does the dissolving.

Calling Every Mixture a Solution

A solution must be uniform. Sand and water do not form a solution.

Mixing Up Saturated and Concentrated

A concentrated solution contains a large amount of solute. A saturated solution has reached its dissolving limit at a particular temperature.

Forgetting Temperature in Solubility

Solubility should be described at a given temperature.

Reading the Meniscus Incorrectly

For colourless liquids, read the bottom of the meniscus at eye level.

Using Mass Instead of Volume in Density

Density requires both mass and volume.

Assuming Shape Changes Density

Changing the shape of the same material does not change its mass or volume. Therefore, its density remains the same.

Important Terms

Quick Revision of The Amazing World of Solutes, Solvents, and Solutions

• A solution is a uniform mixture.
• Salt and sugar form solutions with water.
• Sand and water form a non-uniform mixture.
• The solute dissolves in the solvent.
• A saturated solution cannot dissolve more solute at that temperature.
• An unsaturated solution can dissolve more solute.
• Solubility of most solids increases with temperature.
• Solubility of gases generally decreases with temperature.
• Aquatic organisms depend on dissolved oxygen.
• Density equals mass divided by volume.
• Mass is measured with a balance.
• Liquid volume is measured with a measuring cylinder.
• Read the bottom of the meniscus for colourless liquids.
• Irregular solid volume is measured by displacement.
• Hot air rises because it is less dense.
• Ice floats because it is less dense than water.

Useful Links for Class 8 Science