Topics · Primary Science · Cycles: Boiling, Melting & Freezing
Melting, boiling and freezing: changes of state
Matter changes between solid, liquid and gas when it gains or loses heat. Nothing new is made and nothing is destroyed — the same matter simply arranges itself differently, which is why every change of state can be reversed.
- 333 exam papers analysed
- appears in 90% of P6 Science papers
- ~3 min read
01 Three states, four changes
Solid — fixed shape, fixed volume. Liquid — takes the shape of its container, fixed volume. Gas — fills whatever it is in.
Heat drives every change. Gaining heat moves matter towards gas; losing heat moves it back towards solid.
- Melting — solid to liquid, on gaining heat. Ice melts at 0 °C.
- Freezing — liquid to solid, on losing heat. Water freezes at 0 °C, the same temperature.
- Boiling — liquid to gas, on gaining heat, at a fixed temperature. Water boils at 100 °C.
- Condensing — gas to liquid, on losing heat. Water vapour on a cold window.
- Evaporating — liquid to gas, at any temperature, from the surface only. Washing dries without boiling.
02 Boiling and evaporating are not the same
Both turn liquid into gas, and papers ask the difference directly.
Boiling happens only at the boiling point, throughout the whole liquid, with bubbles, and quickly.
Evaporating happens at any temperature, at the surface only, without bubbles, and slowly.
So a puddle drying on a cool day is evaporating, not boiling. Faster evaporation needs a warmer, windier, drier day or a wider puddle — all of which are standard fair-test questions.
03 The temperature that stops moving
While something is changing state, its temperature stays the same even though heat is still going in. Ice melting sits at 0 °C until the last of it has melted; only then does the water start to warm.
That is why a graph of heating shows flat sections at 0 °C and 100 °C. The flat part is the change of state, and reading it as “the thermometer was broken” or “nothing happened” is the mistake papers are looking for.
04 Mass does not change
Melt 100 g of ice and you get 100 g of water. Boil it away and the steam still weighs 100 g, even though you cannot see it.
This catches people because a change of state looks like something disappearing. It has not — it has become a gas and spread out.
Volume can change; mass does not. Water is unusual in expanding when it freezes, which is why a full bottle of water cracks in a freezer.
05 How to say them
These are words most children have never said out loud, and a word you cannot say is a word you avoid using. Capitals mark the syllable you lean on. Tap the speaker to hear it.
- statemore than onestates
Whether matter is a solid, a liquid or a gas.
- melting point
The temperature at which a solid becomes a liquid. For ice it is 0 °C.
- boiling point
The temperature at which a liquid becomes a gas throughout. For water it is 100 °C.
- evaporatesayi-VAP-uh-rayt
To change from liquid to gas at the surface, at any temperature.
- condensesaykuhn-DENSS
To change from gas to liquid on cooling.
- water vapoursayVAY-puh
Water as a gas. It is invisible — the white cloud above a kettle is tiny droplets, already condensed.
06 What each level is tested on
The same topic, asked differently as it goes up. The percentage is the share of papers at that level in which it appears.
- P4
The three states and their properties, and melting and freezing.
Practise P4 Cycles: Boiling, Melting & Freezing — 10 questions →47% of P4 papers
- P5
Boiling, evaporating and condensing, and the difference between them.
Practise P5 Cycles: Boiling, Melting & Freezing — 10 questions →76% of P5 papers
- P6
Heating and cooling graphs, mass staying the same, and changes of state inside fair tests.
Practise P6 Cycles: Boiling, Melting & Freezing — 9 questions →90% of P6 papers
07 Mistakes worth knowing about
Each of these is wrong. Decide why before you open it.
“Boiling and evaporating are the same thing.”
Boiling is at one fixed temperature, throughout the liquid, with bubbles. Evaporating is at any temperature, from the surface, without bubbles.
“The temperature kept rising while the ice melted.”
It stays at 0 °C until all the ice has melted. The flat part of a heating graph is the change of state.
“The water boiled away, so the mass is gone.”
It became a gas and spread out. 100 g of water becomes 100 g of steam.
“The white cloud above a kettle is steam.”
Steam is an invisible gas. The white cloud is tiny water droplets that have already condensed.
“Water freezes at 0 °C and melts at some other temperature.”
Both happen at 0 °C. It is the same point, crossed in opposite directions.
Now try it
29 questions across 3 levels, marked as you go with the working shown.