Latent heat

Latent heat is the amount of heat needed to change a substance's state (Solid-Liquid-Gas), i.e, water boiling into steam or, the opposite, water freezing into ice. It is latent as it does not show up on a thermometer

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William Cullen (15 April 1710 – 5 February 1790)
Scan of an old picture of Joseph Black
Temperature-dependency of the heats of vaporization for water, methanol, benzene, and acetone.