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Latent heat
Beginner
Energy absorbed or released in a phase change.
Q = mLDuring melting or boiling temperature stays constant while energy changes the phase: Q = mL. Water: L_f = 334 kJ/kg and L_v = 2260 kJ/kg.
Boiling 1 kg of water needs 2.26 MJ, over 5 times the energy to heat it from 0 to 100 °C.
The heat goes into breaking bonds, not raising temperature.
In more detail
Latent heat is the energy absorbed or released during a change of phase at constant temperature, such as melting or boiling. It is measured per kilogram, as the latent heat of fusion or vaporisation.
Real-world uses
- Sweating cools the body because evaporation absorbs latent heat.
- Ice packs stay cold for a long time as the ice absorbs latent heat when melting.
- Ice cubes melting in a drink absorb latent heat without a change in temperature until the ice has all melted.
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Frequently asked questions
Why does temperature stay constant during melting?
Because the added energy is used to break bonds between particles, not to increase their motion.
Why does steam burn worse than boiling water?
Steam releases its latent heat when it condenses on skin, adding to the heating.
Why does steam burn more than boiling water?
Steam releases latent heat when it condenses on the skin, on top of the heat from its temperature.
Related topics in Heat & Thermodynamics
Heat · Specific heat capacity · Molar heat capacity · Calorimetry · Phase diagrams · Triple and critical points