Question

the salt that causes hardness in water also liberates carbon dioxide on heating is

Answer 1

Hello friend,

The salts that causes the hardness in water and also liberate carbon dioxide on heating are Magnesium sulphate and calcium sulphate.

The reason is that these sulphate salts do not decompose on heating and they and therefore the water remains hard. This also the reason why the hardness of water can not be removed by simply boiling it.

I hope you have your answer.

Thanks  

Answer 2

Calcium carbonate generally causes hardness of water when dissolved in it.

Under normal circumstances, Calcium carbonate itself it is insoluble but when reacting with the water it produces CaO and hydrogen gas along with carbon dioxide.

This soda and hydrogen reacts to form calcium bicarbonate that is soluble and causes the hardness of water..  

                        CaCO3 (s) → CaO (s) + CO2 (g)  + H2 (g)

                        CaO (s)+ H2(g)-------------> Ca (HCO3)2