Question

When crystals of copper sulphate are strongly heated the residue obtain is
a) red in colour
b) colourless
c)green in colour
d)blue in colour

Answer 1

b)Colourless
Copper sulphate is a hydrated salt which contains 5 molecules of water of crystallisation, and so is denoted by the chemical formula CuSO₄.5H₂O. These 5 molecules of water of crystallisation are responsible for the blue colour of the salt. However, when copper sulphate is heated for some time, the blue colour disappears and it becomes white-grey in colour. This is because the water molecules are lost on heating and copper sulphate pentahydrate is converted to anhydrous copper sulphate