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Question
'A' is a blue coloured crystalline salt. On heating, it loses blue colour and to give 'B'. When water is added, 'B' gives back to 'A'. Identify A and B, write the equation.
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Solution
Since ‘A’ is a blue coloured crystalline salt, it is CuSO4. 5H2O (Blue vitriol). On heating, it loses all five water molecules and becomes colourless anhydrous CuSO4.
\[\ce{\underset{\text{(A)}}{CuSO4.5H2O} ⇌[heating][cooling]\underset{\text{anhy. copper sulphate} Colourless (B)}{CuSO4 + 5H2O}}\]
When water is added ‘B’ gives back A.
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