Melting, boiling and freezing
Why does boiling water stay at 100 °C when you keep heating it?
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Explain
Heat makes ice melt.
More heat makes water boil.
When you heat ice, it warms up to 0 °C. Then it melts, and the temperature stays flat until it has all melted.
Water warms to 100 °C and then boils. Again the temperature stops rising until it has all turned to steam.
Watch the graph. The flat parts show energy changing the state, and not the temperature.
At melting and boiling points, energy breaks the bonds between particles, and the temperature stays constant. This energy is called latent heat.
Q = m × L. For ice, L = 334 000 J/kg.
Worked example: melting 2 kg of ice takes 2 × 334 000 = 668 000 J.
- Heat melts ice
- Heat warms water
- Heat boils water
Want the full story?
Water boils at 100 °C only at sea-level pressure. On a high mountain, it boils at a lower temperature. Boiling water needs about 2 260 000 J/kg, far more than melting. The sim uses a heater far stronger than a kitchen hob so that you can see it quickly.
The Formula
heat to melt = mass × latent heat
- the heat needed, measured in joules (J)
- the mass, measured in kilograms (kg)
- the latent heat of melting, 334 000 for ice, measured in joules per kilogram (J/kg)
Try the numbers
Heat to melt it: 334,000 J
Try it at home
You need an ice cube, a plate, salt.
You need: an ice cube, a plate, salt.
Hold ice on a plate. Watch it melt.
- Put two ice cubes on a plate.
- Sprinkle salt on one.
- Watch which melts first.
What's happening: salt lowers the melting point of ice, which is why salt is put on icy roads.
Physics is life
Your body
Sweat helps you cool down.
Daily life
Ice keeps drinks cool.
Nature
Lakes freeze from the top.
Space
Comets are dirty snowballs.
Quick check
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Words to know
Melting: A solid turning into a liquid.
- Melting
- A solid turning into a liquid.
- Boiling
- A liquid turning quickly into a gas.
- Evaporation
- A liquid slowly turning into a gas at its surface.