4.21 describe reversible reactions such as the dehydration of hydrated copper (II) sulphate and the effect of heat on ammonium chloride

4.21 •ammonium chloride is a white solid

                  •when heated, it breaks down into the gases ammonia and hydrogen chloride

                             •this is the forward reaction

        •when cooled, it reacts to form ammonium chloride (the solid) again

                   •this is the backward reaction

        •heating hydrated copper (II) sulphate will drive off the crystallisation, leaving behind white anhydrous copper (II) sulphate and water

                   •the reaction is reversed if water is added