Question

why is density of water maximum at 4 degree celsius?​

Answer 1

Water attains its maximum density at about 4oC because it is at that temperature that hydrogen bonds form. The effect of the formation of these bonds is that molecules of water are actually pushed apart and into an alignment scheme (crystals) that prevents them from more closely packing together as in the liquid state.

Between 4oC and 0o,water begins to take on a slushy appearance as these crystals form.