Why Is Ice Less Dense Than Water?

When water is frozen into ice, the change in temperature creates excess hydrogen bonds between water molecules that increase the space between the molecules. The additional space created reduces the density of the water as it freezes, making ice less dense than water. Rather than the molecules packing more tightly together and creating a heavier density, as water freezes the hydrogen bonds increase from a 3.4-molecule bond to a 4-molecule bond, which creates the excess space and reduced density in ice.