Question

why when a zinc rod is kept in a blue copper sulphate solution, the colour disappears ?

Answer 1

zinc is more reactive than copper so when it is dipped is cuso4 soln ...it displaces copper and makes a bond with zn....and copper deposits on rod

Answer 2

Zinc displaces copper from Copper Sulphate solution ($CuSO _{4}.5H_{2} O$ being more reactive than it and forms colourless Zinc Sulphate.
$Zn+CuSO_{4} =ZnSO _{4}+Cu|$
A pink precipitate of copper is also formed.