Class -9th Science Notes

1. What is Matter?

  • Definition: Matter is anything that has mass and occupies space.

  • Examples: Air, water, chair, stone, etc.

  • Matter is made up of tiny particles called atoms or molecules.

2. Characteristics of Particles of Matter

  1. Particles of matter have space between them.

    • Example: Sugar dissolves in water, occupying the spaces between water molecules.

  2. Particles of matter are continuously moving.

    • Example: Diffusion of scent/perfume in air.

  3. Particles of matter attract each other.

    • Force of attraction differs in solids, liquids, and gases.

3. States of Matter

Matter exists mainly in three physical states:

(a) Solids

  • Fixed shape and volume.

  • Particles are tightly packed.

  • Very strong force of attraction.

  • Only vibrational motion.

(b) Liquids

  • No fixed shape but fixed volume.

  • Particles are loosely packed.

  • Moderate force of attraction.

  • Can flow (fluidity).

(c) Gases

  • Neither fixed shape nor volume.

  • Particles are very far apart.

  • Negligible force of attraction.

  • Move randomly at high speed.

👉 Other states (beyond Class 9 basics): Plasma and Bose-Einstein Condensate.

4. Interconversion of States of Matter

  • Matter can change from one state to another by changing temperature or pressure.

(a) Effect of Temperature

  • Melting (Fusion): Solid → Liquid (e.g., ice → water).

  • Freezing: Liquid → Solid (e.g., water → ice).

  • Vaporization (Boiling): Liquid → Gas (e.g., water → steam).

  • Condensation: Gas → Liquid (e.g., steam → water).

  • Sublimation: Solid ↔ Gas (e.g., camphor, dry ice).

(b) Effect of Pressure

  • Increasing pressure → gases liquefy (e.g., LPG, CNG).

5. Important Temperatures

  • Melting Point: The temperature at which a solid turns into liquid. (Ice = 0°C)

  • Boiling Point: The temperature at which a liquid changes into vapour. (Water = 100°C at 1 atm)

  • Latent Heat: Heat energy required to change the state without changing temperature.

    • Latent Heat of Fusion – solid → liquid.

    • Latent Heat of Vaporization – liquid → gas.

6. Diffusion

  • The spontaneous intermixing of particles of two substances due to motion of particles.

  • Faster in gases than liquids; slowest in solids.

7. Evaporation

  • Definition: Slow conversion of liquid into vapour at any temperature below its boiling point.

Factors Affecting Evaporation:

  1. Surface area – larger surface, faster evaporation.

  2. Temperature – higher temperature, faster evaporation.

  3. Wind speed – higher speed, faster evaporation.

  4. Humidity – more humidity, slower evaporation.

Cooling Effect of Evaporation:

  • When a liquid evaporates, it absorbs heat from surroundings, causing cooling.

  • Example: Sweating in summer cools our body.

8. Applications in Daily Life

  • Storing LPG as liquid under pressure.

  • Use of CNG (compressed natural gas).

  • Cooling with wet clothes on the body.

  • Refrigeration based on evaporation.

✅ Summary Points

  • Matter is particulate in nature.

  • Three states: Solid, Liquid, Gas.

  • Interconversion occurs due to changes in temperature and pressure.

  • Evaporation and diffusion are practical applications of particle motion.
Class-9th Biology