Latent Heat
Latent heat (L) is the energy per unit mass absorbed or released by a substance during a first-order phase transition at constant temperature and pressure, Q = mL; it equals the enthalpy change of the transition. For water at atmospheric pressure, the latent heat of fusion is about 334 kJ/kg and of vaporisation about 2,257 kJ/kg at 100 °C.
Curious
Heat a pan of ice and the thermometer stops at 0 °C until all the ice has melted; then it climbs to 100 °C and stops again while the water boils. The heat added during these pauses is called latent (from the Latin for “hidden”) heat, a term introduced by Joseph Black around 1760. It goes into loosening the bonds between molecules rather than speeding them up.
Latent heat works in both directions. Sweat cools you because evaporating water absorbs heat from your skin; steam burns are worse than boiling-water burns because condensing steam releases its latent heat onto the skin. Huge amounts of latent heat released when water vapour condenses power thunderstorms and hurricanes.
Exploring
| Substance | Melting point (°C) | Lf (kJ/kg) | Boiling point (°C) | Lv (kJ/kg) |
|---|---|---|---|---|
| Water | 0 | 334 | 100 | 2,257 |
| Ethanol | −114 | 108 | 78 | ~840 |
| Nitrogen | −210 | 25.7 | −196 | 199 |
| Lead | 327 | 23 | 1,749 | ~870 |
Deep Dive
Thermodynamics of phase change. At a first-order transition, the Gibbs free energies of the two phases are equal, while their entropies differ by ΔS = L/T (see Entropy). The Clausius–Clapeyron relation, dp/dT = L/(TΔv), links latent heat to the slope of the coexistence curve; because ice is less dense than water, the melting curve of water has a negative slope, which is why pressure lowers its melting point slightly (see Rudolf Clausius).
Continuous transitions. Second-order (continuous) transitions, such as the ferromagnetic transition at the Curie temperature (see Ferromagnetism) or the superconducting transition in zero field, have no latent heat; instead the heat capacity shows a jump or divergence (see Specific Heat Capacity).
Earth system. Evaporation and condensation of water move about 80 W/m² of energy, averaged over Earth's surface, from the surface to the atmosphere, a major term in the global energy budget (Trenberth et al., 2009). Latent heat release in rising moist air is the main energy source of tropical cyclones. Estimates of these flows carry uncertainties of several W/m².
Concept Relationship Map
Requires: heat, enthalpy
Enables: phase diagrams, Clausius–Clapeyron equation, refrigeration
Commonly confused with: sensible heat (which changes temperature)
Sources
- Black, J. (1803). Lectures on the Elements of Chemistry. Edinburgh.
- Trenberth, K.E., Fasullo, J.T., Kiehl, J. (2009). “Earth's Global Energy Budget.” Bulletin of the American Meteorological Society, 90, 311–323.
- Atkins, P., de Paula, J. (2014). Atkins' Physical Chemistry (10th ed.). Oxford University Press.