Heating, Cooling & Changing State
PrimaryHot and cold change how matter behaves
Adding heat or taking heat away can change matter from one state to another, without turning it into a different substance at all.
- Melting — a solid gets warm enough to turn into a liquid (ice → water).
- Freezing — a liquid gets cold enough to turn into a solid (water → ice).
- Boiling / evaporating — a liquid gets warm enough to turn into a gas (water → steam).
- Condensing — a gas cools down and turns back into a liquid (steam → water droplets, like on a cold glass).
These changes are still the same substance the whole time — water is water whether it's ice, liquid, or steam. That's why we call these physical changes: nothing new is made, matter just switches between solid, liquid and gas.
Heat makes the tiny bits move faster
Heat gives the tiny bits inside matter more energy to jiggle and move. In a block of ice, the bits are locked in place, barely wiggling. Warm the ice up and the bits jiggle harder and harder until they can finally slip and slide past each other — the ice melts into liquid water. Keep heating, and the bits get so much energy they fly apart completely and escape as steam, a gas.
Cooling does the opposite: take energy away and the bits slow down, get closer together, and eventually lock into place again. That's why a puddle can freeze solid on a cold night, and why steam turns back into water droplets on a cold window.
Try it — watch an ice cube change state
- Put one ice cube on a plate at room temperature and start a timer.
- Check every 10 minutes and draw what you see: is it still solid, turning into a puddle, or all gone?
- When it's fully melted, notice: it's now a liquid — puddle-shaped, taking the shape of the plate.
- For a grown-up-supervised bonus, put a plate of the puddle water outside on a warm sunny day. A few hours later, check whether the water is still there — if it's gone, it evaporated into the air as a gas!
- Put a second ice cube in the freezer inside a cup of warm water and check back in a few hours — did the water freeze solid again?
When ice melts into water, is it still the same substance?
Yes! Melting is a physical change — the water molecules don't change into anything new, they just move around more freely. Ice, liquid water, and steam are all the exact same substance in three different states.
What is happening when water droplets appear on the outside of a cold glass of lemonade?
That's condensing. Invisible water gas (water vapour) in the air touches the cold glass, cools down, and turns back into tiny liquid droplets on the outside.
Would a puddle dry up faster on a hot sunny day or a cold cloudy day?
A hot sunny day! More heat means the water's tiny bits get more energy and evaporate into the air faster, so the puddle disappears sooner.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1You leave an ice cube on a warm table. What happens and why?
- The warm room gives heat to the ice.
- Heat makes the tiny bits move faster, so they can no longer hold a solid shape.
- The ice melts, changing from a solid into liquid water.
Example 2You put liquid water in the freezer overnight. What state is it in the morning?
- The freezer takes heat away from the water.
- The bits slow down and lock into a fixed shape.
- The water freezes into solid ice.
Now you try
Work each one out first, then tap to reveal the worked answer.