Which metals or substances do not melt



As Melting temperature is the temperature at which a substance melts, i.e. changes from a solid to a liquid aggregate state. The melting temperature depends on the substance, but in contrast to the boiling temperature, it only depends very little on the pressure (melting pressure). Melting temperature and pressure are combined as Melting point This describes the state of a pure substance and is part of the melting curve in the phase diagram of the substance.

For pure substances, the melting point is identical to the freezing point and remains constant during the entire melting process. The melting temperature is usually lowered by impurities or in the case of mixtures (lowering of the melting point); in addition, the temperature can rise during the melting process, which leads to a melting point.Area has to do. In the case of amorphous materials such as B. Glasses and some plastics are referred to as the transition temperature. It is also possible to determine a softening temperature. Some substances cannot melt because they decompose chemically beforehand, and others can only sublime under normal conditions.

The melting temperature, along with the density, fracture toughness, strength, ductility, hardness and rigidity, is one of the material properties of a material.

Pressure dependence

The melting point depends on the pressure, but only slightly: To change the melting point by just 1 K, the pressure has to be increased by an average of around 100 atm. It follows that Changes in atmospheric pressure - which can cause noticeable changes in the boiling point - practical does not affect the melting point.

For melting, as for other phase transformations, the Clausius-Clapeyron equation applies, which gives the following temperature change Δ as a good approximation for melting at different pressuresT results in:

Where TM. the melting point, ΔV. the change in volume during melting, Δp the difference between the pressures considered, and HM. the enthalpy of fusion. But since the volume changes ΔV. are relatively small when melting, the pressure dependence of the melting point is also relatively small. For example, if the pressure is increased by 100 atm, the melting point of ice changes by −0.76 K - so ice melts more easily under pressure - while the melting point of carbon tetrachloride increases by +3.7 K. The fact that the melting point of ice or, for example, also of bismuth, decreases when the pressure increases, follows from the fact that its volume is reduced when it melts: Then in the above equation ΔV. and ΔT negative.

importance

The melting point of water is of great importance in biology and for the weather. The determination of the melting point of a substance is also of great importance in qualitative analysis, including identity testing, since many substances can be identified by their melting point. Liquid substances or those with a low melting point are converted into easily crystallizing derivatives: Alcohols can be identified, for example, by measuring the melting points of their esters of nitrobenzoic acid or dinitrobenzoic acid.

The lowering of the melting point (cryoscopy) caused by dissolved substances is the reason why ice can be melted by salt.

determination

An approximate measurement can of course be easily made with a thermometer by melting the sample and reading the melting temperature.

Various methods are available for the exact measurement of the melting point:

See also

Phase diagram, molar lowering of the melting point, boiling temperature

Categories: Threshold (temperature) | Material property | Substance property