Question

why the crystal structure of water have lower density than that of liquid water ?

Answer 1

Ice is less dense than water because the orientation of hydrogen bonds causes molecules to push farther apart, which lowers the density. ... When water freezes, water molecules form a crystalline structure maintained by hydrogen bonding. Solid water, or ice, is less dense than liquid water.

Answer 2

Due to the crystal structure of the solid phase of water, the molecules arrange themselves in a rigid, ordered fashion and end up being, on average, farther apart from each other (than they are in the liquid phase), and thus less dense. Less dense things float because of buoyancy.