(i) Sugar is a complex compound which on heating undergoes decomposition reaction, water gets evaporated thereby leaving behind black carbon' in the test tube.
(ii) The blue colour of copper sulphate is due to its crystalline nature which holds 5 water molecules . (water of crystallisation). On heating, the water molecules disappear and anhydrous copper sulphate (white in colour) is left.
(iii) The green colour of ferrous sulphate crystals is due to the presence of water molecules in each molecule. It loses the water of crystallisation on heating, thus leading to change in colour.