Why is solid water less dense than liquid water

Water is unusual in that its maximum density occurs as a liquid, rather than as a solid. This means ice floats on water. density is the mass per unit volume of a material. For all substances, density changes with temperature The mass of material does not change, but the volume or space that it occupies either increases or decreases with temperature. The vibration of molecules increases as temperature rises and they absorb more energy. For most substances, this increases the space between molecules, making warmer liquids less dense than cooler solids.

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