Class 9 Science Chapter 1: Important Questions with Answers

Anything that occupies space and has mass.

Cubic metre (m³).

The temperature at which a solid melts to a liquid at atmospheric pressure.

The temperature at which a liquid starts boiling at atmospheric pressure.

25 + 273 = 298 K.

Camphor, ammonium chloride (also naphthalene).

Apply pressure and reduce temperature.

Pascal (Pa).

kg m⁻³.

Bose-Einstein Condensate (BEC).

Intermixing of particles of two kinds of matter on their own. Higher temperature raises kinetic energy, so diffusion becomes faster.

Definite shape and distinct boundaries; fixed volume; negligible compressibility — solids are rigid.

It has minute holes with trapped air; pressing expels the air, so it compresses.

Its particles move fast and randomly and hit the walls; this force per unit area is pressure.

Heat absorbed during a change of state without a rise in temperature ("hidden"). Latent heat of fusion: heat needed to change 1 kg of a solid into liquid at atmospheric pressure at its melting point.

Solid CO₂ stored under high pressure. At 1 atm it turns straight into gas without becoming liquid — hence "dry".

Evaporation: any temperature below boiling point, only at the surface. Boiling: at the boiling point, throughout the bulk of the liquid.

Cotton absorbs sweat and exposes it to air; as sweat evaporates it takes latent heat from the body, keeping it cool.

| Property | Solid | Liquid | Gas |
|Shape|definite|not fixed|not fixed|
|Volume|fixed|fixed|not fixed|
|Compressibility|negligible|very low|high|
|Rigid/fluid|rigid|fluid|fluid|
|Space|minimum|intermediate|maximum|
|Attraction|maximum|intermediate|minimum|
|Kinetic energy|minimum|intermediate|maximum|

150 g ice in a beaker with the thermometer bulb touching the ice; low flame. Note temperatures when melting starts and ends, then stir and boil. Temperature stays at 273 K and at 373 K because the heat overcomes attraction (latent heat). Latent heat of fusion: 1 kg solid → liquid at its melting point; of vaporisation: 1 kg liquid → gas at its boiling point (atmospheric pressure).

Change of a liquid into vapour below its boiling point. (1) Larger surface area → faster (spreading clothes); (2) higher temperature → faster; (3) lower humidity → faster (clothes dry slowly on rainy days); (4) higher wind speed → faster (windy day).

Direct change of solid to gas (or back) without becoming liquid. Crushed ammonium chloride/camphor in a china dish, inverted funnel on top, cotton plug in the stem; on slow heating the vapour solidifies on the funnel walls. Raising pressure and lowering temperature liquefies gases; solid CO₂ turns straight to gas when pressure falls.

Matter: chair, air, almonds, cold-drink. Not matter: love, hate, thought, cold, smell, smell of perfume — these are feelings or sensations (the perfume particles are matter; the smell we sense is not).

Hot food's aroma particles have higher kinetic energy, so they diffuse into air fast and far. Cold food's particles move slowly, so diffusion is slow.

Particles of water have space between them and the force of attraction between them is weak.

They are very small; have space between them; are continuously moving (have kinetic energy); attract each other.

Air < exhaust from chimneys < cotton < water < honey < chalk < iron.

Solid: definite shape and volume, negligible compressibility, rigid, least space and KE, maximum attraction. Liquid: fixed volume, no fixed shape, fluid, low compressibility, intermediate. Gas: neither shape nor volume fixed, highly compressible, fluid, maximum space and KE, least attraction.

Rigidity: keeping shape under force (solids). Compressibility: reduction in volume under pressure (gases most). Fluidity: ability to flow (liquids, gases). Filling a container: gases fill it completely. Shape: fixed only for solids. Kinetic energy: gas > liquid > solid. Density: mass per unit volume; generally solid > liquid > gas.

Gas particles move randomly at high speed with negligible attraction, so they spread throughout the vessel.

Gas particles keep hitting the walls; the force per unit area is pressure.

It has a definite shape, distinct boundaries and fixed volume, is rigid and cannot be compressed.

Air particles are far apart with weak attraction; wood particles are closely packed with strong attraction.

Ice has a cage-like arrangement of particles with large empty spaces, so its density is lower than that of water; hence it floats.

(a) 300 − 273 = 27 °C (b) 573 − 273 = 300 °C.

(a) gas (vapour) (b) both liquid and gas — it is the boiling point.

The heat supplied is used to overcome the forces of attraction between particles (latent heat), not to raise the temperature.

Apply high pressure and reduce the temperature.

On a hot dry day humidity is low and temperature high, so water evaporates faster, giving more cooling.

Water seeps through tiny pores and evaporates from the outer surface, taking heat from the pot and water.

These liquids evaporate fast, absorbing heat from the palm.

A saucer gives a larger surface area, so evaporation and cooling are faster.

Cotton clothes — they absorb sweat and expose it to air for evaporation, cooling the body.

(a) 293 − 273 = 20 °C (b) 470 − 273 = 197 °C.

(a) 25 + 273 = 298 K (b) 373 + 273 = 646 K.

(a) Naphthalene sublimes — it changes directly into vapour. (b) Perfume vapour diffuses fast into air and reaches us.

Oxygen < water < sugar.

(a) liquid (b) solid and liquid (melting point) (c) liquid and gas (boiling point).

(a) It has a fixed volume but takes the shape of its container, and it flows. (b) It has a definite shape and fixed volume, and is rigid/incompressible.

Ice takes in extra heat (latent heat of fusion) from its surroundings to melt, before its temperature rises, so it cools more.

Steam — it carries extra energy as latent heat of vaporisation, released when it condenses on the skin.

A: melting (fusion) — solid → liquid; B: vaporisation/boiling — liquid → gas; C: condensation — gas → liquid; D: solidification (freezing) — liquid → solid; E: sublimation — solid → gas; F: deposition (sublimation) — gas → solid.