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Melting point analysis is a common laboratory technique. It is used to identify a sample, establish its purity, and determine its thermal stability. The procedure is relatively simple and straightforward, but requires that the sample be dry and able to absorb heat.
The melting point of a substance is the temperature at which the solid becomes liquid under 1 atmosphere pressure. A pure substance has a single melting point, while substances with two or more components have their melting points over a range of temperatures.
All materials have melting points, although we tend to think of solids as those that are solid at room temperature. Gases, though, become solids at very low temperatures, and liquids also have melting points.
For example, the melting point of water is 0degC. It is at this temperature that the liquid and solid states of water are in equilibrium, meaning that they do not affect each other.
Some materials have higher melting points than others. This is largely because of their lattice energy. Sodium chloride, for example, has much higher lattice energy than hydrogen chloride, and it requires more energy to melt it.
To obtain a melting point, load the sample into a capillary tube (Figure 6.10) with one end sealed and the other open. Load the sample to a height of 22-3mm3mm in the tube (when packed, Figure 6.10d).
Place the sample into a slot behind the viewfinder of a melting point apparatus. There are usually three slots in each apparatus, and multiple melting points can be taken at once after gaining experience with the technique.