When a drop of water is added to white anhydrous copper sulphate, the blue colour returns. What is the main reason?
Answer and explanation
Correct answer: Rejoining of water molecules
The governing concept is reversible hydration of copper sulphate. Heating blue CuSO₄·5H₂O removes its water of crystallisation and leaves white anhydrous CuSO₄. When water is added again, the salt becomes hydrated and the characteristic blue colour reappears. Thus option A is correct. Copper is not burning, sulphur is not melting, and the colour change does not require formation of a new acid.
Frequently asked questions
What is the correct answer to this question?
Rejoining of water molecules
Why is this the correct answer?
The governing concept is reversible hydration of copper sulphate. Heating blue CuSO₄·5H₂O removes its water of crystallisation and leaves white anhydrous CuSO₄. When water is added again, the salt becomes hydrated and the characteristic blue colour reappears. Thus option A is correct. Copper is not burning, sulphur is not melting, and the colour change does not require formation of a new acid.
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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