Science · Matter and heat

Changes of state

Ice melts into water, water evaporates into the sky, and water vapour turns into droplets on a cold glass. Explore melting, freezing, evaporation and condensation, and learn why the temperature stays constant during melting and boiling.

20 min
1 simulation
Mini quiz

Updated: 22 October 2026

Preview

The question

When water starts boiling in a pan on the hob and you keep the heat on, does the temperature of the water keep rising? And if you leave a glass full of ice in a warm room, what happens to the temperature of the water inside while the ice is melting? The answer may surprise you: in both cases, even though you keep supplying energy, the temperature does not change at all for a while.

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Ordering

From ice to steam: the order of heating

Put the cards in order by dragging them or using the up/down buttons. When you are done, submit your answer.

Predict first

If you keep heating ice that is melting, what do you think the thermometer will show?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

The refrigeration cycle of a fridge

Open full page in the Lab →

Fig. 1The refrigeration cycle of a fridge

Tap the door, the thermostat knob or the compressor, or take the fridge apart

Inside 5.2 °C · room 22.0 °C · compressor waiting: the fridge is cold enough

Parts and hidden mechanism

Separation0%

In the opened-up fridge, hover over a part, tap it, or pick it from the list.

Pick an assembly: what it does, and why a real fridge is built that way, appears here.

Inside the fridge

5.2 °C

Room

22.0 °C

Thermostat setting4.0 °CThe compressor starts at 5.0 °C and stops at 3.0 °C

Compressor: waiting: the fridge is cold enough

Time passed: 0 min (1 second on screen = 1 minute)

Heat budget (right now)

Heat taken from inside0 W
Electrical power of the compressor0 W
Heat given to the room0 W

While the compressor rests, the refrigerant does not circulate and no heat is carried.

Enter your own values

Thermostat

Room temperature

Compressor power

Points on the cycle

Compressor outlet · hot vapourpressure 1.85 bar · temperature 22 °C
Condenser outlet · warm liquidpressure 1.85 bar · temperature 22 °C
Capillary outlet · cold liquid + vapourpressure 1.85 bar · temperature 5 °C
Evaporator outlet · cold vapourpressure 1.85 bar · temperature 5 °C

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

On a hot day, if you leave the fridge door open and keep it running, what happens to the room?

Gold tube: hot refrigerant · Purple tube: cold refrigerantSolid line: liquid · Dots: vapourThe numbers come from one simple model: the refrigerant is isobutane (R600a, common in household fridges), and pressures are absolute, in bar, from a curve close to this refrigerant’s real saturation curve. The default values are a 4 °C thermostat, a 22 °C room and a 60 W compressor; you can type your own values in the boxes. A stronger compressor cools the evaporator further and removes more heat, but the electricity it uses grows faster. The fridge and the room are each treated as one body at a single temperature, and heat stored in the tubes is ignored. In a real fridge the values depend on the model, the refrigerant and the surroundings.

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Frequently asked

5 questions

If I turn the hob up all the way, will the water go above 100 °C?

No. At sea level, pure water in an open pan boils at 100 °C and stays at that temperature until it has all turned to vapour. Turning up the heat does not make the water hotter; it only makes it boil faster.

Is the white cloud above boiling water steam?

No. Water vapour is an invisible gas. The white cloud you see is made of tiny water droplets that form when the vapour meets cooler air and condenses. Right above the mouth of the pan, in the part you cannot see, there is pure vapour.

What is inside the big bubbles in boiling water?

The big bubbles that form during boiling are not air but water vapour; because the water starts turning into gas throughout, the bubbles rise from the bottom. The small bubbles you see when you first start heating are dissolved air coming out of the water.

How does wet washing dry without reaching 100 °C?

Evaporation does not only happen at the boiling point; it happens at the surface of a liquid at any temperature. It speeds up with higher temperature, wind and a larger surface area, which is why spreading the washing out and hanging it in a windy spot dries it faster.

Does a substance turn into a different substance when it changes state?

No. Ice, water and water vapour are the same substance; only the arrangement of the particles and the space between them change. That is why a change of state is a physical change: frozen water becomes water again when it melts.