Supercooling

Under cooling referred to in the thermodynamics lowering the temperature of a liquid below the freezing point without solidifying them. Generally, this effect is also known as a supercooled melt. A supercooled liquid or melt thus has a lower temperature corresponds at a given pressure than their state of aggregation. Vaccinating with smallest germs, possibly momentum transfer in the form of vibration, but leads to release the heat of fusion to spontaneous crystallization. Therefore, the expression in the sub-cooling of a metastable state of the substance.

A (mostly very pure ) substance that allows the state of the supercooled melt, thus allows energy storage in the form of heat ( latent heat storage ). This effect is used, inter alia, in hand warmers.

Cause

The reason for this effect is that the change in physical appearance often a crystallization approach is necessary. For example, ice crystals grow only ice crystals or some other arrangements. Are these crystallization approaches does not exist, can not enter a crystallization and thus the change of state, even if it were possible on the temperature here.

As water

Usually, it is assumed the freezing point of water is zero degrees Celsius ( at atmospheric pressure). In fact, the melting point of ice is precisely at this temperature. When freezing is true for kinetic reasons, however, different.

Usually in water by the random motion of molecules always some accumulations (English cluster ) of water molecules in an ordered form available - these are called ice crystal embryos. However, they fall apart just as quickly as they were formed, and not continue to grow because there are for crystals, ie also for ice cream, a certain minimum size ( critical nucleus radius): At -5 ° C, about 50,000 water molecules must assemble ordered before the ice crystal embryo is stable and can continue to grow, at -20 ° C, it must still be a few hundred and random clusters of this size are still pretty unlikely. Only at about -40 ° C is further growth expected, since here already rich cluster of 70 molecules.

The process obtained by the ice crystal embryos of sufficient size that can stabilize and grow to an ice crystal is called ice nucleation. If only water involved, it is homogeneous ice nucleation and this occurs at about -48.3 ° C after hypothermia a.

However, if surfaces with certain characteristics are present ( either as particles in the water, either on the surface of a container ), they have a stabilizing effect on crystal nucleation embryos and is already at higher temperatures (heterogeneous ice nucleation ).