Once a product has been made and purified, its melting temperature (for a solid) or boiling temperature (for a liquid) is measured. Comparing the value with a data book shows whether the product is the expected compound and how pure it is.Melting temperature. A small amount of the dry, powdered solid is packed into a thin capillary tube sealed at one end. The tube is placed in an electric melting temperature apparatus (or attached to a thermometer in an oil bath) and heated slowly, especially near the expected value, so the sample and the thermometer stay at the same temperature. The temperatures at which the solid starts and finishes melting are recorded.A pure solid melts sharply over a narrow range (about 1 °C) that matches the data book value. Impurities lower the melting temperature and make the solid melt over a wider range.Boiling temperature. The liquid is distilled with the bulb of the thermometer at the entrance to the condenser (level with the side arm), so it measures the temperature of the vapour that is distilling over. A pure liquid distils at a constant temperature matching its data book value. Boiling temperature is less useful for checking purity than melting temperature, because several compounds may boil at similar temperatures and the value depends on atmospheric pressure.
Worked example: pure benzoic acid melts at 122 °C. Sample A melts from 121 to 122 °C: it is pure benzoic acid. Sample B melts from 114 to 119 °C: the lower, wider range shows it is impure, so it should be recrystallised again.
Worked example: pure benzoic acid melts at 122 °C. Sample A melts from 121 to 122 °C: it is pure benzoic acid. Sample B melts from 114 to 119 °C: the lower, wider range shows it is impure, so it should be recrystallised again.