Latent heat
The energy required to change the state of a substance (solid ↔ liquid ↔ gas) at constant temperature. Denoted by L, latent heat is measured in J/kg. There are two types: latent heat of fusion (L_f, for melting/freezing) and latent heat of vaporisation (L_v, for boiling/condensing). Energy supplied goes into breaking or forming bonds, not into increasing kinetic energy (temperature).
Formula
Q = mL
Real World
Sweating cools the human body so effectively because water's latent heat of vaporisation (~2.26 MJ/kg) means a tiny amount of evaporated sweat removes a large quantity of heat from the skin.
Exam Focus
State which latent heat you are using (fusion or vaporisation) in every answer — examiners penalise unqualified references to 'latent heat'.
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