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.

\[\ce{CuSO4 * 5H2O -> CuSO4 + 5H2O}\]

Anhydrous copper sulphate turns blue when a few drops of water are added (D) because it rehydrates.

\[\ce{CuSO4 + 5H2O -> CuSO4 * 5H2O}\]