Common Salt (NaCl) is used to preserve foods. Which of the following properties can be used to determine its purity before use?

Analyze the question: You are asked which property can be used to check whether a sample of common salt is pure.

Concept: A pure substance has a sharp, fixed melting point (and boiling point). Impurities change these values. They usually lower the melting point and make the substance melt over a range of temperatures instead of at one exact temperature. This is why melting point is the standard test for purity of solids.

Checking each option:

• A. Solubility in water: Salt dissolves in water whether it is pure or contaminated with other soluble substances. Many impurities are also soluble, so this does not show purity.
• B. Melting point: Pure NaCl melts sharply at about 801 °C. Impurities lower this value and widen the melting range, so a measured melting point reveals how pure the sample is.
• C. Relative density: This can vary with how the sample is packed, its moisture content and the nature of the impurities. It is not a reliable or practical test for purity.
• D. Crystalline nature: Many impurities, such as other salts, are also crystalline. Crystal shape does not tell you whether the sample is pure.

Answer: Melting point is the property that determines purity, so the correct option is B.

Exam tip: Whenever a question asks how to test the purity of a solid, think "sharp melting point". For a liquid, think "fixed boiling point". Impurities lower the melting point and raise the boiling point.