Answer
The formation of ice is dependent on the specific structure of bonding in the water molecule. Salt when added to water( like in ocean) changes this structure and discourages ice formation.
Work Step by Step
A water molecule has a special shape. It has hydrogen attached to oxygen atom at an angle of 105.5 degrees. This allows hydrogen to form the bond with other water molecules, and also with the other highly electronegative atoms. When sodium chloride is dissolved in water, it dissociates into sodium and chlorides ions. The negative ends of water molecules are attracted towards the positively charged sodium ions, and the positive ends of water molecules are disrupted by sodium and chloride ions when sodium chloride is dissolved in water.
Ice forms when molecules form a hexagonal crystal lattice. Water molecules align in a specific way to form ice. When sodium chloride is added, it dissolves and forms hydrogen bonding with water. This alters the inter-molecular hydrogen pattern between water molecules. Sodium and chloride disturb the lattice formation, and hence salt water does not freeze at 0 degrees.
