What is the purpose of the melting point

Melting point

Synonyms: (Tm)
English: melting point

1 definition

The Melting point is the temperature at which a substance changes from a solid to a liquid state of aggregation. It also depends on the pressure and, in addition to a large number of sizes, is a material property. In the case of amorphous materials such as glass and plastic, the term transition temperature is used instead of the melting point. In this case, any substance needs a certain amount of energy in order to overcome the respective inter- and intramolecular / atomic interaction energy in the solid state and thus to transition into a liquid state of aggregation. This energy contribution is almost always supplied thermally, through the temperature.

The melting point is only slightly dependent on the pressure, which can often be neglected. In order to change the melting point by 1 K (or 1 ° C), the pressure would have to be increased by approx. 100 bar, which is very costly. For comparison: the normal atmospheric pressure on earth is around one bar. The melting point is thus determined by a melting temperature and the associated pressure. The melting point is only an indication of pure substances; i.e. substances or compounds that consist of one type of atom (one type of element) or molecules. It should be mentioned here that the term melting temperature only applies to the temperature range under consideration if the substance is not chemically converted (decomposition or disintegration), as in the example of ice melting into water.

Here an H remains2O - molecule chemically unchanged. In most cases, the freezing point of a substance can be equated with the melting point, since the same process only takes place here in reverse order. This material property is often used in qualitative analysis, since many materials have a certain characteristic melting point. This means that substances can be identified very quickly and without great effort.

2 Lowering of the melting point and eutectic mixture

The melting point is often reduced with contaminated substances, which is why one speaks of the melting point lowering. Here, the amount of interaction energy changes in the solid state of aggregation due to foreign atoms or molecules. This is mostly used in melting processes to melt substances at lower temperatures. If there is a mixture of two substances for the purpose of lowering the melting point, the ratio of the two substances to each other determines the extent of the temperature decrease. If an ideal ratio of the components is achieved here so that the temperature cannot be reduced any further, then one speaks of a eutectic mixture or the eutectic point.

3 Calculation of the melting point

As a basic equation, developed by Clausius Clapeyron, the following formula is suitable for calculating the melting point and for many other phase changes. It should be mentioned here that any deviations are due to the fact that the Clausius-Clapeyron equation is based on the laws of the ideal gas!


δT = (Tm* δV * δp) / Hm.

Legend:

  • δT = required temperature change for melting
  • Tm = Temperature of the melting point (mostly tabulated)
  • δV = difference in substance volume between before and after melting
  • Hm = Enthalpy of fusion


The change in pressure is usually insignificant. In addition, a substance usually does not experience a particularly large change in volume when it melts. If these two formula components are neglected, the required melting temperature is thus a quotient of melting point temperature and melting enthalpy.