Criteria of Purity: Melting and Boiling Point Determination

Understanding Purity in Chemistry

In chemistry, the purity of a substance is crucial for its identification and use. Pure substances have consistent properties, such as melting and boiling points, which can be used to determine their purity.

Melting Point (m.p.)

• Definition: The temperature at which a solid turns into a liquid.
• Indicator of Purity: A pure solid has a sharp melting point, meaning it melts at a specific temperature. Impurities usually lower and broaden the melting point range.

Boiling Point (b.p.)

• Definition: The temperature at which a liquid turns into a gas.
• Indicator of Purity: A pure liquid boils at a specific temperature under a given pressure. Impurities can raise or lower the boiling point.

How to Determine Purity Using m.p. and b.p.

To determine the purity of a substance, compare its melting or boiling point with the known values for the pure substance.

 

Example: Boiling Point Determination

A sample of ethanol boils at 78.5°C. The known boiling point of pure ethanol is 78.37°C. The slight increase in boiling point indicates possible impurities.