What Are The Methods Of Testing The Purity Of Substances?

The purity of any substance can be checked through one of the following methods.

Melting Point:
A pure substance has a set and exact melting point.

The identity of a pure solid can be deduced by its melting point. For example, pure stearic acid melts at exactly 70 degrees Celsius. The presence of any impurities will lower the melting point. Also if impurities are present, it will melt over a range of temperatures, say 68 – 70 degrees Celsius.

Boiling Point:
A pure substance has a set and exact boiling point.

The identity of a pure liquid can be deduced by its boiling point. For example, pure ethanol boils at exactly 78 degrees Celsius. The presence of impurities will raise this boiling point. It will also cause the liquid to boil over a range of temperature.

Chromatography:
A pure substance shows only one spot on the chromatogram.

Chromatography is also used to test for purity of substances. It is especially useful with complicated chemicals like medicines or food additives which cannot be either melted or distilled.  For example, by using chromatography, the synthetic dyes contained in foods can be checked to determine, if they are harmless. This technique can detect chemicals even if they are in low concentrations.