Notes Class 9 Science Chapter 1 Matter in Our Surroundings

Notes For All Chapters Science Class 9 CBSE

1. Introduction

Everything in the universe is made of matter.

Matter: Anything that has mass and occupies space (volume).

Examples: Air, water, food, stones, clouds, plants, animals, etc.

Early Indian philosophers classified matter into five elements (Panch Tatva): Air, Earth, Fire, Sky, and Water.

Modern classification: Based on physical properties and chemical nature.

1.1 Physical Nature of Matter

1.1.1 Matter is made up of particles

• Matter consists of tiny particles.
• Evidence: When salt or sugar dissolves in water, it spreads evenly without increasing the water level → shows matter is made of tiny particles.

1.1.2 How small are these particles?

• Experiment with potassium permanganate shows that a few crystals can colour a large amount of water.
• Even after repeated dilution, the colour remains.
• Conclusion: Particles of matter are extremely small — beyond imagination.

1.2 Characteristics of Particles of Matter

1.2.1 Particles have space between them

• Salt, sugar, Dettol, or potassium permanganate dissolve in water — shows spaces exist between particles.
• One type of matter’s particles fit into the spaces of another.

1.2.2 Particles are continuously moving

• Activity: Diffusion of incense stick smell, ink in water, and copper sulphate in hot/cold water.
• Diffusion: Intermixing of particles of two substances on their own.
• Diffusion increases with temperature → higher kinetic energy of particles.

1.2.3 Particles attract each other

Activities with human chains, chalk, rubber band, and water show that:

• There is a force of attraction between particles.
• The strength of this force varies among solids, liquids, and gases.

1.3 States of Matter

Matter exists in three states – Solid, Liquid, and Gas.

1.3.1 The Solid State

• Fixed shape, distinct boundaries, definite volume.
• Negligible compressibility.
• Rigid (maintains shape under force).
• Examples: Book, pen, needle, stick.
• Special cases:
  • Rubber band: Changes shape on stretching but regains it → still solid.
  • Salt, sugar: Fixed shape of crystals even when poured.
  • Sponge: Compressible due to air in holes → still solid.

1.3.2 The Liquid State

• No fixed shape but fixed volume.
• Takes shape of container.
• Can flow → fluid.
• Liquids diffuse into liquids.
• Gases dissolve in liquids (e.g., oxygen, CO₂ in water for aquatic life).
• Rate of diffusion: Liquids > Solids because of more particle movement.

1.3.3 The Gaseous State

• No fixed shape or volume.
• Highly compressible (used in LPG, CNG, oxygen cylinders).
• Diffuse rapidly due to large spaces and high speed.
• Exert pressure on walls of container due to particle collisions.

1.4 Can Matter Change its State?

1.4.1 Effect of Change of Temperature

On heating solids:

• Kinetic energy ↑ → particles vibrate faster → forces weaken → solid melts → liquid.

Melting point: Minimum temperature at which solid melts (for ice = 273 K).

Latent heat of fusion: Heat required to change 1 kg solid → liquid at melting point.

Boiling point: Temperature at which liquid → gas (for water = 373 K).

Latent heat of vaporisation: Heat needed to change 1 kg liquid → gas at boiling point.

Temperature remains constant during melting and boiling because heat is used to overcome forces of attraction, not to raise temperature.

1.4.2 Effect of Change of Pressure

• Compressing gas → particles come closer → can become liquid.
• Example: LPG, CNG.
• Solid CO₂ (Dry Ice): Changes directly from solid → gas on pressure decrease → Sublimation.
• Sublimation: Solid → gas without becoming liquid.
• Deposition: Gas → solid directly.
• Both temperature and pressure determine the state of matter.

1.5 Evaporation

Definition

• Process of liquid changing into vapour below its boiling point.
• Due to some particles at surface having higher kinetic energy to escape into air.

1.5.1 Factors Affecting Evaporation

1. Surface area: More area → faster evaporation.
2. Temperature: Higher temperature → faster evaporation.
3. Humidity: More humidity → slower evaporation.
4. Wind speed: Higher wind → faster evaporation.

1.5.2 How Evaporation Causes Cooling

• During evaporation, particles absorb energy from surroundings → surroundings become cool.
• Examples:
  • Acetone (nail polish remover) feels cool on skin.
  • Sprinkling water on hot floor cools it.
  • Wearing cotton clothes helps absorb sweat → sweat evaporates → body cools.
  • Water droplets form outside a glass of cold water because vapour condenses into liquid.