While matter changes state, heat goes in or comes out without a change of temperature. This hidden heat is the latent heat. The heat for a change of state is the mass times the latent heat per kilogram.
about 25 min1 simulation2 practice questions1 Python program4 questions to check yourself
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The temperature stays the same
Put ice at 0∘C in a beaker, and heat it with a steady flame. Put a thermometer in the ice. The temperature stays at 0∘C until all the ice has melted, though we keep on giving heat. Only when the ice has all melted does the temperature begin to rise.
The same happens when water boils. The temperature stays at 100∘C while the water changes to steam, though we keep on giving heat.
Where does the heat go?
During a change of state, the heat that goes in does not make the particles faster. It is used to break the forces that hold the particles together. The heat seems to be hidden, and this is why it is called latent heat. (Latent means hidden.)
A graph of temperature against the heat added to 1 kg of ice. The line rises from minus 20 degrees to 0, then is flat at 0 degrees for 334 kJ while the ice melts. It then rises to 100 degrees, is flat at 100 degrees for 2260 kJ while the water boils, and then rises again as steam.
The graph shows the heating of 1kg of ice. The flat parts are the changes of state.
You should find that the flat part at 100∘C is almost seven times as long as the flat part at 0∘C: boiling takes far more heat than melting.
The two latent heats
The latent heat of fusion is the heat needed to change 1kg of a solid to a liquid at its melting point. For ice it is 3.34×105J/kg.
The latent heat of vaporisation is the heat needed to change 1kg of a liquid to a gas at its boiling point. For water it is 2.26×106J/kg.
For a mass m, the heat needed is
Q=m×L
where L is the latent heat in J/kg. The same amount of heat comes out when the change goes the other way (freezing or condensation).
Example. How much heat melts 0.5kg of ice at 0∘C?
Q=0.5kg×3.34×105J/kg=1.67×105J
Steam burns more
Steam at 100∘C burns the skin more than boiling water at 100∘C. Both have the same temperature. But each kilogram of steam also carries the latent heat of vaporisation, 2.26×106J, which it gives to the skin when it condenses. So steam gives much more heat.
For the same reason, particles of steam have more energy than particles of water at the same temperature.
Practise with new numbers
Try it in Python
This program finds the heat needed to change 0.5kg of ice at 0∘C to steam at 100∘C.
Remember these
During a change of state, the temperature does not change, though heat goes in or comes out.
Latent heat of fusion of ice: 3.34×105J/kg. Latent heat of vaporisation of water: 2.26×106J/kg.
Q=mL for a change of state.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
How much heat is needed to boil away 0.7kg of water that is already at 100∘C?
Why does the temperature of boiling water not rise when the flame is turned higher?
Why is a burn from steam at 100∘C worse than a burn from water at 100∘C?
How much heat comes out when 0.3kg of water at 0∘C freezes?
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