Water is the same substance whether it is ice cubes clinking in a glass, water sloshing in a bottle, or steam rising off a kettle. What changes is its state — solid, liquid or gas. And the reason all three look and behave so differently comes down to one idea: how the tiny particles inside are arranged and how much they move. Scientists call this the particle model.
Why this matters: the particle model explains melting ice, boiling kettles, misted-up windows and why gases can be squashed. Once you can picture the particles, changes of state stop being facts to memorise and start making sense.
Particles get more spread out and energetic: solid → liquid → gas
The three states
A solid keeps its shape; a liquid pools at the base; a gas fills the whole container.
Solid — particles packed tightly in a regular pattern, only vibrating on the spot. A solid keeps a fixed shape and fixed volume. Picture people squeezed shoulder-to-shoulder in a lift, shuffling but unable to move around.
Liquid — particles still close together but able to slide past one another. A liquid flows and takes the shape of its container, yet keeps a fixed volume. Like a crowd of people milling about a room.
Gas — particles far apart, moving fast in all directions. A gas has no fixed shape or volume — it spreads out to fill its container, and because of all the empty space it can be squashed (compressed). Like people sprinting around an empty sports hall.
Property
Solid
Liquid
Gas
Arrangement
Regular, packed
Close, random
Far apart
Movement
Vibrate only
Slide past
Fast, all directions
Fixed shape?
Yes
No
No
Fixed volume?
Yes
Yes
No
Changes of state
Red = heating up the ladder; blue = cooling back down.
Heating gives particles more energy so they move faster and break free of each other; cooling removes energy so they settle back down. Each change has its own name:
Melting — solid → liquid (heating).
Evaporating / boiling — liquid → gas (heating).
Condensing — gas → liquid (cooling).
Freezing — liquid → solid (cooling).
Working out a change of state.Gas → liquid on cooling is condensing — why a cold window mists up.
A window mists up on a cold morning — what happened? Warm water vapour (a gas) in your breath touched the cold glass, lost energy, and turned into tiny liquid droplets. Gas → liquid on cooling is condensing. To name any change, spot the start state and end state, then check whether heat went in or out.
Common mix-up:melting and freezing get muddled because both involve solids and liquids. Melting is solid → liquid (heating up); freezing is liquid → solid (cooling down). Read the direction carefully before naming the change.
Memory hook: heating climbs the ladder solid → liquid → gas as particles gain energy; cooling slides back down. And “C for Cool, C for Condense” — condensing always needs cooling.