A blue salt becomes white on heating and turns blue again when water is added. Which salt could it be?
Answer and explanation
Correct answer: Copper sulphate
The governing concept is water of crystallisation. Hydrated copper sulphate, CuSO₄·5H₂O, is blue because water molecules are incorporated in its crystal structure. On heating, it loses this water and becomes white anhydrous CuSO₄. Adding water restores the hydrated form and the blue colour. Therefore option B is correct. Sodium chloride and sodium carbonate are not identified by this reversible blue–white change, while calcium carbonate is not a blue hydrated salt.
Frequently asked questions
What is the correct answer to this question?
Copper sulphate
Why is this the correct answer?
The governing concept is water of crystallisation. Hydrated copper sulphate, CuSO₄·5H₂O, is blue because water molecules are incorporated in its crystal structure. On heating, it loses this water and becomes white anhydrous CuSO₄. Adding water restores the hydrated form and the blue colour. Therefore option B is correct. Sodium chloride and sodium carbonate are not identified by this reversible blue–white change, while calcium carbonate is not a blue hydrated salt.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
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