Heat of crystallization

The heat of crystallization (also solidification heat or heat of freezing is called) released when a material 's physical state from liquid to solid changes . Because of the conservation of energy that is at the crystallization or solidification released energy the same magnitude as that for the melting expended energy of the substance ( heat of fusion ), but of opposite sign:

${\displaystyle Q_{\text{kristallisation}}=-Q_{\text{schmelz}}<0}$

The signs refer to the substance, which changes its physical state without changing its temperature ( latent heat ). The surrounding system, on the other hand, experiences exactly the opposite change in energy because it absorbs the energy given off by the solidifying material in the form of heat ( see below). Here the absorbed energy can have an effect as a temperature rise ( sensible heat ). ${\displaystyle Q_{{\text{kristallisation}},{\text{System}}}>0}$

The specific heat of solidification (and thus also the specific heat of fusion), i.e. H. the heat per mass , is dependent on the material (eg list. under heat of fusion ).

The only known material with positive heat of solidification in a particular area is the Helium - isotope ³ He d. i.e. it freezes below 0.3  K when heat is applied ( Pomeranchuk effect ).

application

Ice-covered apple blossoms after frost protection irrigation. The heat of crystallization prevented the frost-sensitive flowers from freezing to death

In practice, the phenomenon of crystallization heat in the can supercooling of water observed: If water is free from vibration and slowly cooled, it freezes at zero  degrees Celsius not . In this way, it can be cooled down to a few degrees below zero (without “attachment points” - similar to “ condensation nuclei ” - even subcooling down to −30 ° C is possible before the water molecules are arranged in a crystal lattice ). Then, for example, a shock can trigger the crystallization spontaneously. The energy released causes the temperature to rise to zero degrees Celsius.

The heat of crystallization is used, for example, in fruit growing : During the night frosts in spring, the flowers are sprinkled with a water mist by means of frost protection sprinkling. The heat of crystallization released when the water freezes keeps the ice temperature at 0 ° C and consequently protects the plant parts enclosed in the ice from frost damage.

Another application of the heat of crystallization is regenerable hand warmers . These are filled with a salt solution, which crystallizes with release of heat when the kinking of a metal plate in the solution causes a shock.