Why does ice have less density?

Answer 1

From Wikipedia article on "ice" An unusual property of ice frozen at a pressure of one atmosphere is that the solid is some 9% less dense than liquid water. Ice is the only known non-metallic substance to expand when it freezes. Ice has a density of 0.9167 g/cm³ at 0 °C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Liquid water is densest, essentially 1.00 g/cm³, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the temperature drops to 0 °C. (In fact, the word "crystal" derives from Greek word for frost.) This is due to hydrogen bonds forming between the water molecules, which line up molecules less efficiently (in terms of volume) when water is frozen. The result of this is that ice floats on liquid water, which is an important factor in Earth's climate (if ice had sunk instead of floating, any body of water would have frozen from the bottom to the surface, killing any fish and other creatures not resistant to freezing temperatures).