Matter in Our Surroundings

• What matter is; its physical states.
• Properties of solids, liquids and gases at the particle level.
• How matter changes state — melting, boiling, sublimation, condensation.
• Evaporation and why it causes cooling.
• Key terms: latent heat, boiling point, melting point.

What is matter?

• Matter: anything that has mass and occupies space (volume).
• Made up of tiny particles (atoms and molecules) that are always in motion.
• Particles have spaces between them and forces of attraction between them.
• Three classical states: solid, liquid, gas.
• Fourth state: plasma (inside stars, lightning) — not covered at this level.

Particle theory of matter

Properties of each state in detail

Solids

• Rigid: cannot be compressed or changed in shape easily.
• High density: particles packed closely.
• Some solids can be moulded under high pressure (e.g., ice, wax).
• Crystal structure: particles in a regular geometric arrangement (salt, diamond).
• Amorphous solids: no regular arrangement (glass, rubber, plastic) — sometimes called pseudo-liquids.

Liquids

• Fluidity: can flow; takes shape of container.
• Surface tension: liquid surface behaves like a stretched membrane (water strider insects walk on water).
• Viscosity: resistance to flow (honey > water > alcohol).
• No fixed shape but fixed volume.

Gases

• Fill entire container: particles move randomly and hit all walls.
• Highly compressible: large spaces between particles (LPG in cylinder compressed to liquid).
• Low density.
• Pressure: particles constantly hitting container walls = gas pressure.
• Diffusion: gases spread quickly (smell of food, perfume).

Interconversion of states

Matter can change from one state to another by adding or removing heat energy:

SOLID ←→ LIQUID ←→ GAS

Examples of sublimation:

• Dry ice (solid CO₂) → CO₂ gas (used in fog machines, cold storage).
• Iodine (purple solid) → purple vapour on heating.
• Camphor (naphthalene balls) → vapour.
• Ammonium chloride sublimes and is used to separate it from other salts.

Melting point and Boiling point

• Melting point: temperature at which a solid converts to liquid at standard pressure.
  • Ice: 0°C = 273 K.
  • Pure substances have a sharp melting point — used to check purity.
• Boiling point: temperature at which liquid converts to gas throughout its volume.
  • Water: 100°C = 373 K at 1 atm pressure.
  • Effect of pressure:
    • Increased pressure → raises boiling point (pressure cooker: water boils at ~120°C → food cooks faster).
    • Decreased pressure → lowers boiling point (on mountains, water boils at < 100°C → food takes longer to cook).

Latent heat

• When a substance is changing state, temperature does not change even though heat is being added/removed.
• This heat is used to break/form intermolecular bonds, not raise temperature.
• Latent heat of fusion: heat absorbed to melt 1 kg of solid → liquid at melting point.
  • Ice: 336,000 J/kg (336 kJ/kg).
• Latent heat of vaporisation: heat absorbed to vaporise 1 kg of liquid → gas at boiling point.
  • Water: 2,260,000 J/kg (2,260 kJ/kg) — very high → why steam burns are worse than boiling water burns.

Evaporation

• Evaporation: liquid changing to vapour at temperatures below the boiling point.
• Occurs only at the surface (unlike boiling which occurs throughout).
• Faster-moving surface particles escape into air.

Factors affecting evaporation rate

Evaporation causes cooling

• Particles that escape are the fastest-moving (highest kinetic energy) ones.
• Average kinetic energy of remaining liquid decreases → temperature falls.
• Examples:
  • Sweating cools the body (sweat evaporates → takes heat from skin).
  • Earthen pots (matka) keep water cool (water seeps through tiny pores, evaporates → cools remaining water).
  • Acetone/spirit on skin feels cold (evaporates quickly).
  • Desert coolers work by evaporation.

Kelvin scale

• SI unit of temperature: Kelvin (K).
• K = °C + 273
• Absolute zero = 0 K = −273°C (theoretically lowest possible temperature; all particle motion stops).
• Water freezes: 273 K; Water boils: 373 K.

Plasma — the fourth state

• At extremely high temperatures (thousands to millions of °C), atoms lose electrons → mixture of free electrons and ions.
• Plasma: ionised gas; most common state of matter in the universe (stars are plasma).
• Examples on Earth: lightning, neon signs, fluorescent tube lights, plasma TV screens.

• Compare solids, liquids and gases on: arrangement, movement, compressibility.
• What is sublimation? Give two examples.
• What is latent heat of vaporisation? Why do steam burns hurt more than boiling water burns?
• List four factors that increase the rate of evaporation.
• Why does an earthen pot keep water cool? Explain using evaporation.

Open the Practice tab for graded questions on Matter in Our Surroundings.