Why does water expand when frozen when almost all decreases in volume?

Answer 1

An H2O molecule consists of an O atom with two H atoms, one on each side of the O. The 3 atoms are not in a straight line; there's an obtuse angle between the two O-H bonds. Hydrogen bonds, previously mentioned, are strange critters. When liquid water freezes, the adjacent water molecules take up positions with a lot of empty space between them, unlike most but not all other freezing liquids. Some metal alloys also expand when they solidify. Under high pressure, there are at least 8 different crystalline structures which ice can have that I've heard of, depending on temperature and pressure. As you'd expect, the higher the pressure, the more compact and dense the structure.

Answer 2

Because water is not linear and because the electron affinity of O is so large compared to H, the water molecule has a very large dipole moment. Which leads to hydrogen bonding. The hydrogen bonding creates a structure when water is frozen that is less dense then the water in liquid phase. hence, it expands upon freezing.

Answer 3

When water freezes to make ice, it creates a very regular packing due to hydrogen bonds. When this packing occurs it creates these large holes which would otherwise be filled when water is a liquid. So these added holes would make water expand.

there is a link in the bottom to the packing of ice

Answer 4

This is because of the unique crystal packing of water which arises because of the strong hydrogen bonding interactions that are present in water molecules.

Answer 5

The crystal structures formed cause the water molecules to be held further apart form one another than when it is liquid.