latent heat

latent heat

latent heat, heat change associated with a change of state or phase (see states of matter). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a gas, or the reverse of either of these changes. It is called latent because it is not associated with a change in temperature. Each substance has a characteristic heat of fusion, associated with the solid-liquid transition, and a characteristic heat of vaporization, associated with the liquid-gas transition. The latent heat of fusion for ice is 80 calories per gram (see calorie). This amount of heat is absorbed by each gram of ice in melting or is given up by each gram of water in freezing. The latent heat of vaporization of steam is 540 calories per gram, absorbed during vaporization or given up during condensation. For a substance going directly from the solid to the gas state, or the reverse, the heat absorbed or given up is known as the latent heat of sublimation.

Characteristic amount of energy absorbed or released by a substance during a change in physical state that occurs without a change in temperature. Heat of fusion is the latent heat associated with melting a solid or freezing a liquid. Heat of vaporization is the latent heat associated with vapourizing a liquid or condensing (see condensation) a vapour. For example, when water reaches its boiling point and is kept boiling, it remains at that temperature until it has all evaporated; all the heat added to the water is absorbed as latent heat of vaporization and is carried away by the escaping vapour molecules.

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In thermochemistry, latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase state(i.e. solid, liquid, or gas), – also called a phase transition.

The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden. The term is now dated, replaced by "enthalpy of transformation".

Two latent heats (or enthalpies) are typically described: latent heat of fusion (melting), and latent heat of vaporization (boiling). The names describe the direction of heat flow from one phase to the next: solid → liquid → gas.

The change is endothermic, i.e. the system absorbs energy, when the change is from solid to liquid to gas. It is exothermic (the process releases energy) when it is in the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains more water vapor than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

Latent heat equation

The equation for latent heat is:

Q = mL ,
where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in joules),
m is the mass of the substance,
L is the specific latent heat for a particular substance (J kg-1).

In other words, specific latent heat is found when energy is divided by mass.

Table of latent heats

Latent heats and change of phase temps of common fluids and gases
Substance Latent Heat
Fusion
J/g
Melting
Point
°C
Latent Heat
Vaporization
J/g
Boiling
Point
°C
Alcohol, ethyl 108 -114 855 78.3
Ammonia 339 -75 1369 -33.34
Carbon dioxide 184 -57 574 -78
Helium     21 -268.93
Hydrogen 58 -259 455 -253
Lead 24.5 372.3 871 1750
Nitrogen 25.7 -210 200 -196
Oxygen 13.9 -219 213 -183
R134a   -101 215.9 -26.6
Toluene   -93 351 110.6
Turpentine     293  
Water 334 0 2500 (at 0oC) 100

References

See also

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