Why the density of ice is less than that of water

Water reaches its maximum density at 4°C (40°F). As it cools further and freezes into ice, it actually becomes less dense. On the other hand, most substances are most dense in their solid (frozen) state than in their liquid state. Water is different because of hydrogen bonding.

Ice is less denser than water because in ice the molecules arrange themselves in a rigid tetrahedral structure due to which cage like spaces remain in their bonding. But water molecules remain in linear bonding form. As the volume of ice becomes greater, it is less denser.