Phase Change & Latent Heat

Phase change (transition) occurs at a FIXED temperature for a pure substance (e.g., water melts at 0°C, boils at 100°C at 1 atm).

Latent heat L: energy absorbed/released per kg during phase change. Q = m·L.

Two main types: LATENT HEAT OF FUSION (L_f) for melting/freezing; LATENT HEAT OF VAPORIZATION (L_v) for boiling/condensing.

Water: L_f = 334 kJ/kg, L_v = 2260 kJ/kg.

During phase change, temperature stays CONSTANT even as heat is added — all energy goes into breaking molecular bonds.

Sublimation: solid → gas (skipping liquid), e.g., dry ice. Deposition: gas → solid.

Pressure changes phase-transition temperatures (P-T phase diagram).

Sweating cools you because evaporation of sweat absorbs L_v from your skin.