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What is the Melting Point?

The melting point is the temperature at which a phase change occurs between the solid and liquid phases. The temperature at which this is reversed, which is usually approximately the same, is called the freezing point, although the tendency of liquids to be super-cooled means that this is a less predictable property. At the melting point an equilibrium exists between the ordered crystalline state and random liquid states. It means that the number of molecules that gain energy to become liquid is the same as the number of molecules that loose energy to form solid. Usually there is a melting-point range where the initial temperature at which the first solid crystals are formed from drops of liquid is lower than the temperature at which the solid entirely disappears into liquid form.

The melting points of two substances can be used to separate them if there is a sufficient difference between them. It can also be used as a diagnostic to identify compounds, or to assess purity, because the melting point of a pure substance is always constant. If we add an impurity to a pure substance its melting point will vary according to the amount of impurity added. Usually a pure substance melts at higher temperature and has a narrower melting point range than a sample which contains impurities.

If we continue adding impurity to a pure substance a minimum melting point is eventually reached. The ratio of pure substance and impurity at this lowest melting point is known as the eutectic point. For example, if salt is added to ice it lowers the melting point and consequently the ice melts. The melting point of ice at 1 atmosphere of pressure is to 0 °C (32 °F, 273.15 K), also known as the ice point, but this can be depressed by several degrees by adding salt.

Melting Point and Intermolecular Forces

When we heat a substance it gains energy. As the solid gains energy the average inter-molecular distance and inter-atomic distance start to increase. The energy required to change from solid to liquid phase, and vice versa, is known as the heat of fusion. The melting of the substance is due to the increase in distance between the molecules and atoms of the substance and the stronger these forces the greater is the heat of fusion and usually the higher is the melting point. This is the reason why iron has a higher melting point than ice. The forces between the molecules and atoms of iron are stronger as compared to that of ice. The melting point of iron is 1538°C whereas that of water is 0°C.

Measuring the Melting Point

The apparatus for measuring the melting point of crystalline solids consists of an oil bath with a window to observe the solid. A special type of test tube for this purpose is called a Thiele tube that is designed to allow a controlled convection current of heated oil. The sample is placed in a glass capillary tube attached to a thermometer and immersed in the oil bath which is then carefully heated. It is then a case of observation using a magnifier to detect when melting occurs.

More sophisticated laboratory devices use an electrically heated block and an optical device to automatically detect the melting of the solid.

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