The hardness caused by dissolution of soluble bicarbonates of calcium or magnesium in water is called temporary hardness can be removed simply by heating. So, such type of hardness is called temporary hardness. The hardness caused by dissolution of soluble sulphates and chlorides of magnesium or calcium in water is called permanent hardness. This type of hardness can't be removed by the simple method and needs special chemical methods. Therefore, such type of hardness is called permanent hardness. In the water molecule, central atom oxygen undergoessp3hybridization. This leads to the tetrahedral shape of the molecule in which two H atoms should lie at two the concerns of the tetrahedron and the rest of two concerns are occupied by two lone pairs of electrons of the oxygen atom. But as lone par-lone pair repulsion is greater than that between lone pair-bond pair and bond pair-bond pair, the tetrahedral shape is distorted. Thus, HOH bond angle decreases from 109.5° (which is a regular tetrahedral bond angle) to 104.5°. So, the water molecule has a distorted tetrahedral structure. It has an angular or bent (V-shaped) structure if atomic nuclei only are considered.