Module 5 §1: Thermal Physics · Year 2
Specific Latent Heat
Revision notes on Specific Latent Heat for the OCR A-level Physics specification (H556). Free to read, with 3 practice questions in the app.
Specific latent heat (L) — the energy required to change the state of 1 kg of a substance without any change in temperature. Its unit is J kg⁻¹.
E = mL
Specific latent heat of fusion — for melting or freezing, between solid and liquid.
Specific latent heat of vaporisation — for boiling or condensing, between liquid and gas.
The word latent means hidden: energy is supplied, yet the thermometer does not move. The energy is going into the potential energy of the particles as they are pulled apart, not into their kinetic energy.
Example: for water, the specific latent heat of fusion is about 3.3 × 10⁵ J kg⁻¹ and of vaporisation about 2.3 × 10⁶ J kg⁻¹ — roughly seven times larger. Melting only has to loosen the lattice enough for particles to slide past one another; boiling has to separate them completely and push back the atmosphere to make room for the vapour.
Why sweating cools you — evaporating sweat takes its latent heat of vaporisation from your skin. Each kilogram that evaporates removes about 2.3 MJ, which is why a breeze, which speeds evaporation, feels so much cooler than still air at the same temperature.
On a heating curve — the flat sections are the changes of state. Their length is proportional to the latent heat involved, so for water the boiling plateau is far longer than the melting plateau at the same rate of heating. The sloping sections are governed by specific heat capacity, and their gradient is inversely proportional to c: the larger the specific heat capacity, the more slowly the temperature climbs.
3 Practice questions on Specific Latent Heat
Multiple choice and calculations for this topic are in the app, one question at a time. Written answers are marked against the specification and you get the mark scheme with the feedback.
Practise Specific Latent Heat