Criteria of Purity of a Substance

Criteria of Purity of a Substance

The melting point and boiling point of a substance indicates the purity of a substance. Every pure compound has a definite melting point or boiling point. If the substance melts at lower temperature or boils at a higher temperature then it is impure. The impurities depress the melting point and elevate the boiling point.

Melting Point: The pure solid substance melts sharply at a definite temperature while as impure substance will have a lower and indefinite melting point. In the laboratory it is generally determined in sulphuric acid as shown in figure.

melting point

Boiling Point: A liquid boils at a temperature at which its vapour pressure becomes equal to the atmospheric pressure. The presence of impurities raises the boiling point of the liquid. The apparatus which is generally-used for the determination of boiling point is shown in figure

boiling point