The purity of a solid sample can best be determined by its

1. Analyze the question You are asked which physical property is the best test of purity for a solid sample.

2. Explain the concept A pure substance has a sharp, definite melting point, meaning it melts at one specific temperature (or over a very narrow range of about 1°C). If impurities are present:

• The melting point is lowered.
• The melting range becomes wider, so the solid melts gradually instead of sharply.

This is why chemists compare the measured melting point of a sample with the known value for the pure substance.

3. Go through the options

• A. Boiling point: This is mainly used for liquids. Many solids decompose before they boil, and it is not practical for testing a solid.
• B. Melting point: This is the standard purity test for solids. A sharp melting point at the expected temperature means the sample is pure.
• C. Conductivity: This tells you whether a substance is a metal, an electrolyte or an insulator. It does not reliably show purity.
• D. Solubility: This depends on the solvent and conditions. It is not a precise or standard test of purity.

4. Answer The correct option is **B.

5. Exam tip Remember: "Solid → Melting point, Liquid → Boiling point." Pure substances have sharp fixed points, and impurities lower the melting point and raise the boiling point.