What is the physical or chemical property that causes hardness?

Answer by  shobby (42)

Hardness is of two types - Temporary hardness - Due to presence of Mg and Ca hydrogencarbonates it can be removed by boiling. 2. Permanent Hardness due to presence of Mg and Ca in the form of chlorides and sulphates in water, it can removed by treatment with washing soda.

Answer by  rendo (66)

Hardness has everything to do with the crystal structure of the solid. Some solids have very dense structures, with strong bonds making them hard.