What is hard water?
When water is referred to as 'hard' this simply means, that it contains higher minerals than ordinary water. These are especially the minerals calcium and magnesium. Increased dissolved calcium and magnesium result in an increase in hardness.
Magnesium and calcium are positively charged ions. Because of their presence, other positively charged ions will dissolve less easily in hard water than in water that does not contain calcium and magnesium. This is the reason that soap doesn't dissolve very well in hard water