Water occurs in the form of snow, as water in rivers, lakes, sea, etc. and as vapour in the atmosphere. Water is a covalent compound made up of two hydrogen atoms linked with one oxygen atom through covalent bonds.

Because of the large electronegativity of oxygen, water molecule is highly polar. It has partial negative charge (δ) on the oxygen atom and partial positive charge (δ+) on the hydrogen atom. An electrostatic attraction between H of one molecule with O of other molecule results in the formation of inter-molecular hydrogen bonds.

Structure of Ice

The remarkable characteristic of water is that in solid form, it is less dense than liquid form. Consequently, an ice cube floats on water. Water molecules are joined together in an extensive three dimensional network in which oxygen atom is bonded to four hydrogen atoms, two by hydrogen bonds and two by normal covalent bonds, in a near tetrahedral hydrogen bonded structure, which has got open spaces. This is responsible for low density.