Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents and Solutions Notes: A Complete Study Guide

Ever wondered why sugar disappears when you stir it into water? Or why soft drinks have dissolved gases in them? The answer lies in the knowledge of solutes, solvents and solutions.In this chapter, learners will investigate the mixing of different substances, factors that influence their solubility and how everyday properties such as mass, volume and density are measured and used in science.

These Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents and Solutions notes will help students to build their understanding and revise well for their exams with the help of clear explanations, relevant examples, important formulae and practical applications.

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

What is a Solution?	Solubility
Solute and Solvent	Factors Affecting Solubility
Types of Solutions	Density
Saturated and Unsaturated Solutions	Floating and Sinking of Objects

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

What is a Solution

A solution is a homogeneous mixture . It is formed when one substance dissolves completely in another substance . In a solution the particles are evenly distributed so that the mixture looks like one substance throughout.

Examples of Solutions

• Sugar dissolved in water
• Salt dissolved in water
• Lemon juice
• Vinegar in water
• Soft drinks

A solution contains two important components:

1. Solute
2. Solvent

Solute and Solvent

What is a Solute

A solute is the substance that gets dissolved in a solvent.

Examples

Solution	Solute
Sugar water	Sugar
Salt water	Salt
Lemon water	Lemon juice

What is a Solvent:

A solvent is the substance that dissolves the solute.

Examples

Solution	Solvent
Sugar water	Water
Salt water	Water
Lemon water	Water

Water is known as the universal solvent because it can dissolve a large number of substances.

Solutions Formed by Mixing Liquids

When two liquids are mixed, the liquid present in a smaller amount is generally considered the solute, while the liquid present in a larger amount acts as the solvent.

Example

In a mixture of alcohol and water:

• Alcohol is Solute
• Water is Solvent

What is Solubility

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

Different substances have different solubilities.

Examples

• Sugar is highly soluble in water.
• Sand is insoluble in water.
• Oxygen dissolves slightly in water.

Factors Affecting Solubility

Several factors affect how much solute can dissolve in a solvent.

Temperature

Temperature has a significant effect on solubility.

For Solids: The solubility of most solids increases with temperature.

Example: More sugar dissolves in hot water than in cold water.

For Gases: The solubility of gases decreases as temperature increases.

Example:Cold water contains more dissolved oxygen than warm water.

Nature of the Solute and Solvent

The type of solute and solvent determines whether they can dissolve in each other. Substances with similar chemical properties usually dissolve more easily.

• Stirring (Agitation)

Stirring helps the solute particles spread throughout the solvent more quickly. It increases the speed of dissolving but does not change the maximum amount that can dissolve.

• Particle Size of the Solute

Smaller solute particles dissolve faster because they have a larger surface area exposed to the solvent. Finely powdered substances generally dissolve more quickly than larger pieces.

• Pressure (for Gases)

Pressure mainly affects the solubility of gases in liquids. Increasing the pressure allows more gas to dissolve, while reducing the pressure decreases gas solubility.

Saturated and Unsaturated Solution

Unsaturated Solution

A saturated solution contains the maximum amount of solute that can dissolve at a given temperature. No more solute can dissolve in the solution.

Example: When sugar continues to remain undissolved at the bottom of a glass, the solution has become saturated.

Unsaturated Solution

An unsaturated solution contains less solute than the maximum amount it can dissolve.More solute can still dissolve in it.

Example: Sugar water in which additional sugar can still dissolve.

Measuring Mass: 

Mass is the amount of matter present in an object.

Units of Mass

• Gram (g)
• Kilogram (kg)

Instruments Used: Digital weighing balance and Two-pan balance

Mass and weight are not the same.

Mass	Weight
Amount of matter in an object	Force with which Earth attracts an object
Measured in grams or kilograms	Measured in newtons
Constant everywhere	Changes with gravity

Measuring Volume

Volume is the amount of space occupied by an object or substance.

SI Unit of Volume: Cubic metre (m³)

Common Units

• Cubic centimetre (cm³)
• Litre (L)
• Millilitre (mL)

Relationships

• 1 L = 1000 mL
• 1 cm³ = 1 mL
• 1 dm³ = 1 L

Measuring Cylinder

A measuring cylinder is commonly used to measure the volume of liquids.

Features

• Transparent cylindrical container
• Marked with volume graduations
• Available in different capacities

Meniscus

When a liquid is poured into a measuring cylinder, it forms a curved surface called a meniscus.

Rule for Reading Volume

• Colourless liquids to Read at the bottom of the meniscus.
• Coloured liquids to Read at the top of the meniscus.

Always keep your eyes at the same level as the meniscus for accurate readings.

Measuring Volume of Regular Solids

The volume of regular solids can be calculated using mathematical formulas.

Cuboid

$\text{Volume} = l \times w \times h$

Where:

• l = Length
• w = Width
• h = Height

Example

Length = 25 cm

Width = 18 cm

Height = 2 cm

Volume = $25 \times 18 \times 2$

Volume =  $900\ cm^3$

Measuring Volume of Irregular Solids

Objects such as stones and keys do not have regular shapes.Their volume is measured using the water displacement method.

Procedure

1. Fill a measuring cylinder with water.
2. Note the initial volume.
3. Immerse the object completely.
4. Note the final volume.
5. Calculate the difference.

Formula

$\text{Volume of Object} = \text{Final Volume} - \text{Initial Volume}$

Example

Initial Volume = 50 mL

Final Volume = 55 mL

Volume=55−50 

Volume =  $5\ cm^3$

What is Density?

Density is the mass present per unit volume of a substance.Density helps us compare how closely particles are packed together.

Formula for Density

$\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

Unit of Density

$g/cm^3$or$kg/m^3$

Example of Density Calculation

Mass = 16.4 g

Volume = 5 cm³

Density = $\frac{16.4}{5}$

Density=516.4 

Density =  $3.28\ g/cm^3$

Density helps in:

• Identifying substances
• Determining purity
• Designing ships and boats
• Studying Earth's layers
• Understanding floating and sinking

Effect of Temperature on Density

Generally, density decreases when temperature increases.

Reason

When heated:

• Particles move farther apart.
• Volume increases.
• Mass remains unchanged.

Therefore,

Density= $\frac{Mass}{Volume}$

As volume increases, density decreases.

Example

Hot air is less dense than cold air.This principle is used in hot-air balloons.

Effect of Pressure on Density

Pressure affects different states of matter differently.

Gases

Increasing pressure:

• Decreases volume
• Increases density

Liquids

Liquids are nearly incompressible.Pressure has only a small effect on their density.

Solids

Pressure has a negligible effect on density.

Why Does Ice Float on Water?

Ice floats because it is less dense than liquid water.

When water freezes:

• Its particles arrange themselves farther apart.
• Volume increases.
• Density decreases.

As a result, ice becomes lighter than water and floats on the surface.This property helps aquatic organisms survive in cold regions.

Floating and Sinking

Whether an object floats or sinks depends mainly on its density.

Condition	Result
Density less than water	Floats
Density greater than water	Sinks

Examples

Floats: Ice, wood and bamboo raft

Sinks:Stone, Iron nail and Metal key

Applications of Density in Daily Life

• Design of ships and submarines
• Hot-air balloons
• Oil floating on water
• Ice floating in lakes
• Identification of minerals and metals
• Geological studies of Earth's layers