The melt is the phase transition of a solid material or solid substance mixture in the liquid aggregate state , mostly due to supply of heat and / or pressure reduction. If a pure substance is melted at constant pressure , the melting temperature present is clearly determined. Melting temperature and pressure are collectively referred to as the melting point . During the transition, the temperature remains constant, and all the heat supplied is invested (as melting enthalpy ) in changing the physical state. For example, a water / ice mixture has a temperature of 0 ° C at normal pressure.
The opposite of melting is solidification .
Substances with a melting temperature (at normal pressure) below the usual room temperature are sometimes referred to as “thawing”; its opposite is then "freezing".
Mixtures of substances
In the case of mixtures of substances, the melting process is more complicated, since the composition of the solid and liquid phase is added as a further degree of freedom . The melting process no longer takes place at a constant temperature, but in a temperature interval, the melting range. The eutectic mixture , which behaves like a pure substance, is an exception . The behavior of a mixture during the melting process can be represented by a melting diagram . The liquid phase of a substance or mixture of substances is called the melt .
In the case of some substances, the transition from the solid to the liquid state takes place via intermediate phases, so-called mesophases . Liquid crystals form smectic and nematic phases between the solid and liquid state . First the long-range order in the crystal is given up, whereby a short-range order remains. In the opposite case, the short-range order is first dissolved in plastic crystals , whereby a long-range order is retained. Adamantane can be mentioned as an example , where the highly symmetrical ("spherical") molecules in the plastic state have corresponding degrees of freedom of rotation, but the space in the crystal lattice is retained.
Melting by increasing the pressure
In general, the liquid of a substance has a lower density than its solid state. With such substances, reducing the pressure favors melting. In the case of substances whose liquid state has / can have a higher density than their solid state, the substance can be liquefied by increasing the pressure; the best-known example is water, which has its highest liquid density at approx. 4 ° C and expands when it freezes.
A supercooled melt is understood to be a liquid that is below its melting temperature and therefore in a physical state that does not correspond to its state of equilibrium . This effect is also simply called hypothermia and is used in some latent heat storage systems.
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