Notes Chapter 7 Science Curiosity Class 8 CBSE Board

Notes For All Chapters – Science Curiosity Class 8

Particulate Nature of Matter

Introduction

• Matter exists in different forms: solids, liquids, gases.
• Questions to think: Why can we pile sand but not water? How does air give weight to a balloon?

7.1 What is Matter Composed of?

• Chalk experiment: breaking into smaller pieces → shows matter is made of small constituent particles.
• Sugar in water: dissolves, not seen but tasted → proves particles exist and mix.
• Matter is composed of extremely small particles.
• Particles cannot be seen even with ordinary microscope.
• Interparticle spaces: sugar particles fill gaps between water particles.

7.2 What Decides Different States of Matter?

• Particles held by interparticle attractions (forces of attraction).
• Strength depends on distance between particles.
• Decides state of matter (solid, liquid, gas).

Our Scientific Heritage

• Acharya Kanad introduced idea of Parmanu (atom) in Vaisheshika Sutras.

7.2.1 Solid State

• Particles tightly packed, strong attractions.
• Solids have fixed shape and volume.
• Particles vibrate but cannot move freely.
• On heating → vibrations increase → at melting point, solid changes to liquid.
• Melting point = minimum temperature at which solid melts at normal pressure.

Examples:

• Ice: 0°C
• Urea: 133°C
• Iron: 1538°C

7.2.2 Liquid State

• Particles less tightly packed than solids.
• Liquids have no fixed shape, take shape of container.
• Liquids have fixed volume.
• Interparticle attractions weaker than solids.
• On heating → liquid boils, changes to gas at boiling point.
• Evaporation: slow process at all temperatures (only surface).

7.2.3 Gaseous State

• Particles very far apart, negligible attraction.
• Gases have no fixed shape or volume.
• Occupy full space available.
• Particles move freely in all directions.
• Liquids + gases = fluids (they can flow).

7.3 Interparticle Spacing

• Solids: minimum spacing.
• Liquids: more spacing than solids.
• Gases: maximum spacing.
• Sugar in water: dissolves by occupying spaces.
• Sand in water: does not dissolve → settles at bottom.
• Interparticle spaces are not filled with air, they are empty.

7.4 Movement of Particles

1. In Liquids:

• Potassium permanganate spreads in water.
• Spreads faster in hot water than cold → heat increases motion.

2. In Gases:

• Fragrance of incense stick spreads in room.
• Shows gas particles move freely in all directions.

3. Everyday Example: Soap particles remove oil stains by attaching to oil on one side and water on the other.

Thermal Energy and Change of State

• Thermal energy decides state of matter.
• Solids: low energy, strong attractions, only vibrations.
• Liquids: more energy, particles move within space.
• Gases: high energy, particles free, negligible attractions.
• Heating increases energy → changes state (melting, boiling, evaporation).

Particle Nature of Three States (Summary Table)

Snapshots (Key Points)

• Matter is made of very small particles.
• Particles have interparticle attractions.
• Solids: strongest forces, fixed shape & volume.
• Liquids: weaker forces, fixed volume, no fixed shape.
• Gases: negligible forces, no fixed shape & volume.
• Heating increases particle motion and changes state.