What's covered
Key facts
Particle theory
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.
States of matter
Changes of state are physical changes, not chemical changes: no new substance is made and the change can be reversed.
Particle theory
Density depends on how closely packed the particles are; solids are densest (particles closest), gases least dense (particles furthest apart).
States of matter
Changes of state: solid → liquid (melting), liquid → solid (freezing/solidifying), liquid → gas (evaporation/boiling), gas → liquid (condensation), solid → gas (sublimation).
Particle theory
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, due to their random motion.
States of matter
Gases have no fixed shape or volume and expand to fill any container; their particles are far apart, move rapidly, and in random directions.
Particle theory
Diffusion is faster at higher temperatures (particles move faster) and over shorter distances; lighter particles diffuse faster than heavier ones.
States of matter
Liquids have a fixed volume but take the shape of their container; their particles are close together but free to move past each other.
Particle theory
Gases are easily compressed because there is a large amount of space between the particles; solids and liquids are almost incompressible.
States of matter
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.
Practice questions with answers
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 have a fixed shape?
- •Particles are all very heavy
- •Particles do not move at all
- •Solids contain no particles
- ✓Strong forces hold particles in place
What happens to particle motion when heated?
- •Particles become heavier
- •Particles get smaller in size
- ✓Particles move faster
- •Particles stop moving completely
Why does a gas fill its whole container?
- •Gas is attracted to container walls
- •Gas is too hot to settle
- ✓Particles move freely in all directions
- •Particles repel each other strongly
What is diffusion?
- •Particles collide and bond together
- •Particles dissolve from solid into liquid
- •Particles move from low to high concentration
- ✓Particles spread from high to low
Which states of matter are there?
- •Hot and cold
- •Just solid and liquid
- ✓Solid, liquid, gas
- •Wet and dry
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