What's covered
Key facts
Brownian motion — the random jiggling of small particles (e.g. pollen grains in water) — is direct evidence for the particle model; it is caused by random collisions with smaller, invisible fluid particles.
Changes of state are physical changes, not chemical changes: no new substance is made and the change can be reversed.
Density depends on how closely packed the particles are; solids are densest (particles closest), gases least dense (particles furthest apart).
Changes of state: solid → liquid (melting), liquid → solid (freezing/solidifying), liquid → gas (evaporation/boiling), gas → liquid (condensation), solid → gas (sublimation).
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, due to their random motion.
Gases have no fixed shape or volume and expand to fill any container; their particles are far apart, move rapidly, and in random directions.
Diffusion is faster at higher temperatures (particles move faster) and over shorter distances; lighter particles diffuse faster than heavier ones.
Liquids have a fixed volume but take the shape of their container; their particles are close together but free to move past each other.
Gases are easily compressed because there is a large amount of space between the particles; solids and liquids are almost incompressible.
Particles in all three states are always moving — vibrating in fixed positions in a solid, moving past each other in a liquid, and moving rapidly and randomly in a gas.
Sample questions
A taste of the 25 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What does the particle model state?
- •All matter is made of one element
- ✓All matter is made of tiny particles
- •Matter is made of cells
- •Particles are visible to the eye
Why do solids and liquids expand when they are heated?
- •New particles are created by the heat
- •Their particles grow larger in size
- •Their particles join together into bigger ones
- ✓Their particles move faster and spread further apart
Which states of matter are there?
- •Hot and cold
- •Just solid and liquid
- ✓Solid, liquid, gas
- •Wet and dry
What happens to particle motion when heated?
- •Particles become heavier
- •Particles get smaller in size
- ✓Particles move faster
- •Particles stop moving completely
Why can gases be compressed much more easily than solids or liquids?
- •Gas particles are joined by strong bonds
- •Gas particles are much smaller than other particles
- •Gas particles stop moving when squashed
- ✓There is a large amount of space between gas particles
How are particles arranged in a solid?
- •Far apart and moving freely
- •In long parallel chains
- •Loosely packed and free to flow
- ✓Tightly packed in fixed positions
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