Question

The metal salt A is blue in colour. When salt A is heated strongly over a burner, then a substance B is eliminated and a white powder C is left behind. When a few drops of a liquid D are added to powder C, it becomes blue again. What could be A, B, C and D

Answer 1

Copper sulphate crystals (A) are blue and have the chemical formula CuSO₄.5H₂O. When copper sulphate crystals are heated strongly, they lose all the water of crystallisation (B) and form anhydrous copper sulphate (C), which is white.
     A                        C            B
CuSO₄.5H₂O  →  CuSO₄  + 5H₂O
Anhydrous copper sulphate turns blue on addition of a few drops of water (D) because it gets hydrated again.
        C              D
CuSO₄ + 5H₂O → CuSO₄.5H₂O