Class 9 Science – Chapter 1: Matter in Our Surroundings (Full Detailed Notes)

Class 9 Science – Chapter 1: Matter in Our Surroundings (Detailed Notes)

1. Introduction to Matter

• The entire universe is made up of matter. Everything that we see, touch, smell, or feel is matter.
• Matter is defined as anything that has mass and occupies space.
• Examples of matter include air, water, wood, iron, oxygen, sand, etc.
• Light, sound, and heat are not considered matter because they do not occupy space or have mass.

2. Physical Nature of Matter

(a) Matter is made up of particles

• All matter is composed of extremely small particles that are invisible to the naked eye.
• These particles have space between them and are in constant motion.
• Experiment: When potassium permanganate is dissolved in water, it spreads evenly and retains its color even after multiple dilutions. This shows that particles are very small and continue to spread.

(b) Particles of matter have space between them

• There is space between the particles of matter. This space is called intermolecular space.
• Evidence: When sugar or salt is dissolved in water, it disappears, indicating particles of one substance occupy spaces between the particles of another.

(c) Particles of matter are continuously moving

• Particles of matter are always in motion due to kinetic energy.
• This movement increases when temperature rises.
• Example: Diffusion of perfume in a room or spreading of ink in water.

(d) Particles of matter attract each other

• There is a force of attraction between particles of matter, called intermolecular force.
• This force is strongest in solids, less in liquids, and negligible in gases.
• That’s why solids have a fixed shape, while gases do not.

3. States of Matter

Matter mainly exists in three states: solid, liquid, and gas.

(a) Solids

• Have a definite shape and volume.
• Particles are tightly packed in a fixed pattern.
• Cannot be compressed easily.
• Very strong intermolecular forces and very little space between particles.

(b) Liquids

• Have a definite volume but no fixed shape. They take the shape of the container.
• Particles are less tightly packed and can move past each other.
• Liquids are less compressible compared to gases.

(c) Gases

• Do not have a definite shape or volume.
• Particles are far apart and move freely.
• They are highly compressible and fill the entire space of a container.

4. Can Matter Change Its State?

• Yes, matter can change its state by changing temperature or pressure.

(a) Effect of Temperature

• When a solid is heated, its particles gain energy and start vibrating more strongly.
• At a certain temperature, they overcome the force of attraction and become liquid (melting).
• Further heating changes the liquid into gas (boiling).

(b) Effect of Pressure

• Increasing pressure can compress gas into liquid.
• Example: Liquefied Petroleum Gas (LPG) and Compressed Natural Gas (CNG).

5. Latent Heat

• Latent heat is the heat energy required to change the state of a substance without changing its temperature.
• Latent Heat of Fusion: Heat required to convert solid into liquid.
• Latent Heat of Vaporization: Heat required to convert liquid into gas.

6. Sublimation

• Sublimation is the process in which a solid directly changes into gas without becoming a liquid.
• Examples: Camphor, Naphthalene, Iodine, Dry Ice (solid CO₂).

7. Evaporation

• Evaporation is the process by which a liquid changes into vapour at a temperature below its boiling point.

Factors Affecting Evaporation:

• Surface Area: More area = faster evaporation.
• Temperature: Higher temperature = faster evaporation.
• Humidity: Lower humidity = faster evaporation.
• Wind Speed: Higher wind = faster evaporation.

Evaporation causes cooling

• When a liquid evaporates, it takes heat from its surroundings causing a cooling effect.
• Example: Wet clothes dry faster under fan because evaporation increases due to wind speed.

8. Interconversion of States of Matter

Change	Process	Example
Solid to Liquid	Melting	Ice to Water
Liquid to Gas	Boiling	Water to Steam
Gas to Liquid	Condensation	Steam to Water
Liquid to Solid	Freezing	Water to Ice
Solid to Gas	Sublimation	Camphor, Dry Ice

9. Key Definitions

• Matter: Anything that has mass and occupies space.
• Inter-particle Space: The space between particles of matter.
• Intermolecular Force: The force of attraction between particles.
• Latent Heat: The hidden heat energy required for a change in state.
• Sublimation: Direct change from solid to gas and vice versa.
• Boiling Point: Temperature at which a liquid changes to gas.
• Melting Point: Temperature at which a solid changes to liquid.