Phase Change & Latent Heat
Ice → water → steam: T plateaus during phase change.
Key Notes
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.
Formulas
Heat for phase change
L in J/kg. No T change during phase transition.
Latent heat of fusion (water)
Energy to melt 1 kg of ice at 0°C.
Latent heat of vaporization (water)
Energy to vaporize 1 kg of water at 100°C.
Total heat (with phase change)
Heat ice → water → steam combines specific and latent heats.
Important Points
During phase change, T stays CONSTANT — all heat goes to phase transition.
L_v >> L_f for water: vaporization requires ~7× more energy than melting (per kg).
Why does sweating cool you? Sweat evaporation absorbs L_v from your body.
Steam at 100°C burns worse than boiling water at 100°C because of the EXTRA L_v released as it condenses on skin.
Pressure cookers raise boiling T (higher P shifts phase boundary).
Phase diagrams plot P vs T; triple point and critical point are key features.
Phase Change & Latent Heat notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.