In this video, I explain how both temporary and permanent hard water forms and the techniques used to remove it. Dissolved calcium hydrogencarbonate is responsible for temporary hardness. Rainwater is naturally acidic and thus reacts with calcium carbonate in rocks to form this calcium hydrogencarbonate which is soluble. Boiling water can help to decompose this, producing calcium carbonate and it is this which forms the recognizable layer of limescale in kettles, irons and hot water pipes. Permanent hardness is caused by Calcium and Magnesium Sulphates, and requires greater intervention than simply boiling.