| Dictionary: latent heat |
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| Chemistry Dictionary: latent heat |
Symbol L. The quantity of heat absorbed or released when a substance changes its physical phase at constant temperature (e.g. from solid to liquid at the melting point or from liquid to gas at the boiling point). For example, the latent heat of vaporization is the energy a substance absorbs from its surroundings in order to overcome the attractive forces between its molecules as it changes from a liquid to a gas and in order to do work against the external atmosphere as it expands. In thermodynamic terms the latent heat is the enthalpy of evaporation (ΔH), i.e. L=ΔH=ΔU+pΔV, where ΔU is the change in the internal energy, p is the pressure, and ΔV is the change in volume.
The specific latent heat (symbol l) is the heat absorbed or released per unit mass of a substance in the course of its isothermal change of phase. The molar latent heat is the heat absorbed or released per unit amount of substance during an isothermal change of state.
| Britannica Concise Encyclopedia: latent heat |
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| Geography Dictionary: latent heat |
The quantity of heat absorbed or released when a substance changes its physical state at constant temperature, e.g. from a solid to a liquid at its melting point, or from a liquid to a gas at its boiling point. The release of latent heat of condensation in the rising air of a
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The amount of heat which is absorbed or evolved in changing the state of a substance without changing its temperature, e.g., in freezing or vaporizing water.
| Columbia Encyclopedia: latent heat |
| Science Dictionary: latent heat |
The heat released or absorbed when matter undergoes a change of phase (see phases of matter). If the heat is given off during the change from a liquid to a solid, it is called heat of fusion. If it is given off during the change from a gas to a liquid, it is called heat of vaporization.
| Wikipedia: Latent heat |
The expression latent heat refers to the amount of energy released or absorbed by a chemical substance during a change of state that occurs without changing its temperature, meaning a phase transition such as the melting of ice or the boiling of water.[1][2] The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden.
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Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting) and latent heat of vaporization (boiling). These names describe the direction of energy flow when changing from one phase to the next: solid → liquid → gas.
In both cases, the change is endothermic, meaning that the system absorbs energy on going from solid to liquid to gas. The change is exothermic (the process releases energy) for 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.
The specific latent heat is the amount of energy required to convert 1 kg (or 1 lb) of a substance from solid to liquid (or vice-versa) without a change in the temperature of the surroundings -- all absorbed energy goes into the phase change -- is known as the specific latent heat of fusion. Likewise, the amount of energy required to convert 1 kg (or 1 lb) of a substance from liquid to gas (or vice-versa) without a change in the external temperature is known as the specific latent heat of vaporization for that substance. Tables of values for the two specific latent heats are published; values for some common substances are given below.
The latent heat for a different mass of the substance can be calculated using the equation:
where:
In other words, specific latent heat is found when energy is divided by mass, or as represented as
.
The following table shows the latent heats and change of phase temperatures of some common fluids and gases.
| Substance | Latent Heat Fusion kJ/kg |
Melting Point °C |
Latent Heat Vaporization kJ/kg |
Boiling Point °C |
|---|---|---|---|---|
| Alcohol, ethyl | 108 | -114 | 855 | 78.3 |
| Ammonia | 339 | -75 | 1369 | -33.34 |
| Carbon dioxide | 184 | -78 | 574 | -57 |
| Helium | 21 | -268.93 | ||
| Hydrogen(2) | 58 | -259 | 455 | -253 |
| Lead[3] | 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 | 2260 (at 100oC) | 100 |
To calculate the latent heat of condensation in water in the temperature range from −40 °C to 40 °C the following empirical cubic function can be used:
with an determination coefficient of R2 = 0.999988
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| evaporation (physics) | |
| Black, Joseph (British chemist) | |
| decalescence (metallurgy) |
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