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?
Copper sulphate pentahydrate (CuSO4.5H2O) crystals (salt A) are blue in colour; intense heating causes loss of all water (substance B) and leaves white anhydrous copper sulphate (salt C) behind.
CuSO4.5H2O A ⟶CuSO4 B +5H2O C
Addition of a few drops of water (liquid D) turns the anhydrous copper sulphate into blue coloured copper sulphate pentahydrate.
CuSO4 C+5H2O D ⟶CuSO4.5H2O A