Question

why density of water decreases when freezed below 3.98 degree celsius

Answer 1

The clusters still have empty spaces, so they decrease the density of the liquid. The molecules of the water are closer together, and this increases the density of the liquid. As the temperature of warm water decreases, the water molecules slow down and the density increases.

Answer 2

because as water is freezed it turns into solid and the molecules of the liquid get rigid as well as compressed and hence the density of water decreases