Why does freezing water expand?

Water is certainly strange stuff. Most liquids which turn into solids as they cool tend to contract as part of the cooling process.

But water reaches a point where it becomes like a slushy drink (just before it turns into solid ice) and becomes less dense. The magic figure is 4 degrees Celsius, and up to that point the water has behaved normally and steadily contracted as it cooled. But when it hits 4 degrees it starts to expand until the most dramatic expansion (about 9 per cent) which occurs at 0 degrees C when it finally turns to ice. Water is the only substance where the solid is less dense than the liquid. If it weren’t, then ice cubes would sink to the bottom of your drink.

The chemistry bit

Each water molecule consists of 2 hydrogen atoms bonded to 1 oxygen atom by hydrogen bonding, which is a form of bonding that occurs between hydrogen in 1 molecule and a negative ion in another molecule. In liquid water, as the molecules move freely past each other, bonds are formed and fractured quite easily. But by the time water has cooled to 4 degrees, the molecules energy has dropped sufficiently for movement to slow, so that each H2O molecule forms more stable hydrogen bonds with up to 4 fellow molecules.

At freezing point, the H2O molecules are lined up in a frozen crystal lattice and the molecules are held rigidly apart. That means more empty space between

Molecules, so the frozen water occupies more room than the water from which it came.