On heating blue vitriol, its blue colour turns white. On adding a few drops of water, the blue colour returns. What is the main reason?
Answer and explanation
Correct answer: Loss and regain of water of crystallisation
Blue vitriol is hydrated copper(II) sulphate, CuSO₄·5H₂O. Heating removes its water of crystallisation and produces anhydrous copper sulphate, CuSO₄, which is white: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Adding water rehydrates the salt and restores the blue hydrated form. No copper metal or sulphur dioxide is produced, and the observation is not simply complete dissolution. Thus option A explains both colour changes.
Frequently asked questions
What is the correct answer to this question?
Loss and regain of water of crystallisation
Why is this the correct answer?
Blue vitriol is hydrated copper(II) sulphate, CuSO₄·5H₂O. Heating removes its water of crystallisation and produces anhydrous copper sulphate, CuSO₄, which is white: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Adding water rehydrates the salt and restores the blue hydrated form. No copper metal or sulphur dioxide is produced, and the observation is not simply complete dissolution. Thus option A explains both colour changes.
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.
