Latent Heat

Latent heat is the energy a phase change absorbs or releases while the temperature holds still. Melting 1 kg of ice at 0 °C soaks up 334 kJ — enough to heat that same water from 0 °C to 80 °C — yet the thermometer never moves until the last crystal is gone. Boiling is costlier still: vaporizing a kilogram of water takes about 2256 kJ, more than five times the energy needed to warm it from ice-cold to boiling.

Joseph Black coined the term in 1762 — latent means hidden, because the heat disappears into the phase change instead of the temperature reading. The physics runs everyday life: sweat cools you as it evaporates, steam scalds far worse than boiling water because it dumps its latent heat on condensing against skin, and every refrigerator moves heat by evaporating and condensing a working fluid in an endless loop.

One quantity, three notations, depending on whose book you are holding. Physics writes it L, as here, and splits it into L_f for fusion and L_v for vaporization. Engineering and every steam table print it h_fg, where f is saturated fluid and g is saturated gas — so h_fg = h_g − h_f is the gap between the two columns, which is exactly why it shrinks to nothing at the critical point where those columns meet. Chemistry writes it as an enthalpy of vaporization per MOLE rather than per kilogram, so its numbers look nothing like these until you divide by the molar mass. Same energy, three addresses.