Latent heat is energy transferred as heat when a substance undergoes a phase transition, such as melting, vaporization, or condensation. For a pure substance changing phase in equilibrium at constant pressure, this transfer occurs without a change in temperature. The adjective latent, meaning hidden, reflects the absence of a temperature rise or fall despite heat being absorbed or released. The term can describe the total transferred energy or, more commonly in tabulated data, the energy required per unit mass. (openstax.org)
Definition and units
The heat associated with transforming a mass (m) completely from one phase to another is
[ Q=mL, ]
where (Q) is the transferred heat, (m) is mass, and (L) is the specific latent heat. In the International System of Units, (Q) is measured in joules and (L) in joules per kilogram. For partial transformation, (m) denotes only the mass that changes phase. Tables generally give positive magnitudes; the direction of heat transfer distinguishes absorption from release. (openstax.org)
In chemistry, phase-change energies are also expressed per mole as molar enthalpies of transition, measured in joules per mole. These must be multiplied by the amount of substance rather than its mass. Latent heat contrasts with sensible heat, which changes temperature within a phase. When the specific heat capacity (c) is approximately constant, sensible heating is represented by (Q=mc\Delta T). Calculations spanning several phases add the individual heating and transformation contributions. (openstax.org)
Types and representative values
The principal forms are distinguished by the phases involved:
- Latent heat of fusion: energy absorbed when a solid melts. Freezing releases the corresponding amount under the same conditions.
- Latent heat of vaporization: energy absorbed when a liquid becomes vapor. Condensation releases the corresponding amount.
- Latent heat of sublimation: energy absorbed when a solid changes directly into vapor. The reverse process, deposition, releases energy. (openstax.org)
For water at approximately standard atmospheric pressure, melting ice at (0^\circ\mathrm{C}) requires about (334\ \mathrm{kJ,kg^{-1}}), whereas vaporizing liquid water at (100^\circ\mathrm{C}) requires about (2256\ \mathrm{kJ,kg^{-1}}). Thus, transforming one kilogram at those respective transition temperatures requires about 334 kJ or 2.26 MJ, excluding any preliminary heating. These values describe particular conditions, not universal constants independent of temperature and pressure. (openstax.org)
Evaporation can occur below the boiling point. It still requires energy, which may be supplied by the remaining liquid and its surroundings, producing cooling. Water’s specific latent heat of vaporization is greater at lower temperatures than at its normal boiling point. (openstax.org)
Thermodynamic interpretation
In thermodynamics, the latent heat of a constant-pressure transition corresponds to the difference in specific enthalpy between the phases, provided pressure–volume work is the only work involved:
[ L=\Delta h=\Delta u+p\Delta v. ]
Here (u) is specific internal energy, (p) is pressure, and (v) is specific volume. Consequently, latent heat is not simply the change in internal energy: during vaporization, part of the supplied energy performs expansion work against the surroundings. This follows from the first law of thermodynamics. (web.mit.edu)
For an equilibrium transition carried out as a reversible process at absolute temperature (T), the specific entropy difference satisfies
[ L=T\Delta s. ]
Temperature is expressed in kelvins in this relation. Latent heat therefore measures an enthalpy difference and is directly related to an entropy difference between coexisting phases. (ocw.mit.edu)
The Clapeyron equation connects the latent heat to the slope of a phase-coexistence curve:
[ \frac{dp}{dT}=\frac{L}{T\Delta v}. ]
For liquid–vapor equilibrium, this relates the variation of saturation pressure with temperature to the vaporization enthalpy and volume change. Treating the vapor as an ideal gas, neglecting liquid volume, and approximating (L) as constant yields the familiar integrated Clausius–Clapeyron relation. (web.mit.edu)
Heating curves and microscopic meaning
An equilibrium heating curve for a pure substance at fixed pressure contains rising portions within single phases and plateaus during melting or boiling. Along a plateau, supplied heat changes the proportions of the two phases rather than their common temperature. Once transformation is complete, further heating raises the temperature of the resulting phase. Such curves provide a basis for calorimetry calculations involving ice, liquid water, and steam. (openstax.org)
Microscopically, melting and vaporization alter particle arrangements and interactions. Vaporization requires energy to overcome attractions between molecules, while melting changes the organized structure of a solid. The transferred energy therefore need not produce a temperature increase. Latent heat is not a separate substance concealed inside matter, but an energy difference associated with the transformation. (openstax.org)
Not every phase transition has latent heat. Continuous transitions lack the finite entropy discontinuity associated with thermal latent heat, although their heat capacities may show pronounced anomalies. Thus, “phase transition” and “latent-heat process” are not interchangeable descriptions. (web.mit.edu)
Environmental and technological roles
In the water cycle, evaporation absorbs energy at a surface, while subsequent condensation releases energy into the atmosphere. Condensation heating can increase the buoyancy of rising air and contribute to cloud development. Evaporating rain can instead cool the air through which it falls. These processes couple water transport with atmospheric energy transfer. (weather.gov)
Phase-change materials exploit melting and solidification to store and release thermal energy near a chosen transition temperature. Building applications can absorb daytime heat gains and release stored energy during cooler periods, shifting thermal loads. Their operation depends on both latent-heat capacity and suitable transition temperatures. (energy.gov)
Vapor-compression heat pumps use evaporation and condensation of a refrigerant to exchange heat at different temperatures. The evaporator absorbs heat as refrigerant vaporizes, while the condenser releases heat as it returns to liquid; mechanical input enables the overall transfer from a cooler region to a warmer one. (betterbuildingssolutioncenter.energy.gov)