Latent heat - sennetsu

Japanese: 潜熱 - せんねつ

Generally, when heat is applied to an object, the temperature of the object rises. However, when heat is applied to ice at 1 atmosphere, the temperature rises until it reaches 0°C, but after that, no matter how much heat is added, the temperature does not rise until the ice melts. Heat that does not increase the temperature when applied is called latent heat. Latent heat occurs when a state changes, such as from a solid to a liquid. The latent heat associated with melting is called heat of fusion, and the latent heat associated with evaporation is called heat of evaporation. Additionally, the latent heat that occurs when a solid directly changes into a gas, known as sublimation, is called heat of sublimation.

Latent heat can exist not only in phase transitions between solids, liquids, and gases, but also in phase transitions between solids, in which case the latent heat is called transition heat. Generally, a phase transition accompanied by latent heat is called a first-order phase transition. The ferromagnetic-paramagnetic transition of a magnetic material is a second-order phase transition, in which the specific heat increases at the transition point, but there is no transition heat. A first-order phase transition is accompanied by a change in volume. Latent heat is generally expressed as the amount of heat per unit mass, but can also be expressed as the amount of heat per mole.

Accurate measurement of heat of vaporization L is difficult due to errors in calorimetry, but by using the Clausius-Clapeyron equation, it can be indirectly measured from the change in vapor pressure P with temperature and the change in volume ΔV during evaporation. This equation is dP/dT=L/(T ΔV )
This relationship can also be applied to other phase transitions.

[Aki Ono]

weather

In meteorology, this refers to the latent heat that accompanies a change in the state of water. The air above the tropical ocean is hot and humid, but when this air cools down by rising, the water vapor contained in it condenses and releases a large amount of latent heat. This latent heat is the main source of energy for tropical cyclones.

[Hiroshi Matano]

[Reference] | Heat of vaporization | Phase transition

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

一般に物体に熱を加えれば物体の温度は上昇する。しかし、1気圧で氷に熱を加えると、0℃になるまでは温度が上がるが、その後はいくら熱を加えても、氷が融解してしまうまでは温度が上がらない。このように、加えても温度の上昇を伴わない熱を潜熱という。潜熱は、固体から液体といった状態の変化に際しておこる。融解に伴う潜熱を融解熱、蒸発に伴う潜熱を蒸発熱という。また、固体から気体に直接変化する昇華の際の潜熱を昇華熱という。

　潜熱は、固体、液体、気体の間の相転移の場合に限らず、固体の間の相転移でも存在する場合があり、この場合の潜熱を転移熱という。また一般に潜熱を伴う相転移を一次相転移という。磁性体の強磁性体―常磁性体転移は二次相転移で、転移点で比熱は大きくなるが、転移熱は存在しない。なお、一次の相転移では体積の変化を伴う。潜熱は一般に、単位質量当りの熱量で表すが、1モル当りの熱量で表すこともある。

　蒸発熱Lなどの精密な測定は、熱量測定に伴う誤差のため困難であるが、クラウジウス‐クラペイロンの方程式を用いれば、蒸気圧Pの温度による変化と、蒸発の際の体積の変化ΔVから間接に測定できる。この方程式は
　　dP/dT=L/(TΔV)
である。なお、このような関係は他の相転移の場合にも適用できる。

［小野　周］