Specific latent heat — Physical education, 14–17 years
How a substance can absorb or release energy while its temperature stays unchanged during a change of state.
Energy for changing state
When a substance melts or boils, supplied energy separates its particles instead of raising the temperature. When it freezes or condenses, energy is released while the temperature stays fixed. Specific latent heat is the energy needed to change the state of 1 kg of a substance.
Why temperature can pause
A thermometer alone can make heating look puzzling: energy enters, but the reading pauses at melting or boiling. The problem is that temperature describes particle motion, while a state change also changes particle spacing and attractions. Latent heat accounts for this hidden energy transfer.
Melting ice
Ice has a specific latent heat of fusion of 334,000 J/kg. To melt 0.50 kg at its melting point, use energy = mass × specific latent heat. Energy = 0.50 × 334,000 = 167,000 J. This energy melts the ice; it does not raise the temperature while melting.
The temperature plateau
A common mistake is to keep adding a temperature rise while ice is melting, as if every joule made particles move faster. This is reasonable because heating normally raises a thermometer reading. During a state change, however, the energy is used to rearrange particles, so the temperature remains constant until the change finishes.
Cooling with evaporation
Evaporation removes the faster-moving particles from a liquid, taking energy with them and cooling what remains. This is why sweat can cool skin and why evaporation is used in refrigerators and cooling towers. It works best when air can carry the vapour away.
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