Sophie McIntosh, of Tokomairiro High School, asks :-

Why does water take up more space when it turns to ice?

Kathryn McGrath, a chemist at Otago University, responded.

Unlike most materials, the solid form of water occupies a greater volume of space, about one-ninth more, than an equal mass of the liquid form). Why?

A water molecule is formed when two atoms of hydrogen bond covalently with an atom of oxygen. In a 'covalent bond', electrons are shared between atoms. In water, the sharing is not equal.

The oxygen atom attracts the electrons more strongly than the hydrogen. This gives water an asymmetrical distribution of elecrical charge. Molecules that have ends with partial negative and positive charges are known as polar molecules.

The positive regions in one water molecule will attract the negatively charged regions in other water molecules.

In a 'hydrogen bond', a hydrogen atom is shared by two other atoms. The donor is the atom to which the hydrogen is more tightly linked. The acceptor (having a partial negative charge) is the atom that attracts the hydrogen atom. Water is therefore said to be 'cohesive'.

Hydrogen bonds are much weaker than covalent bonds. However, when a large number of hydrogen bonds act in unison they will make a strong contributory effect. This is the case in water where hydrogen bonds dominate many of its properties.

Liquid water has a partially ordered structure. Hydrogen bonds are constantly being formed and breaking up. Ice, on the other hand, has a rigid lattice structure.

In liquid water, each water molecule is hydrogen bonded to approximately 3.4 other water molecules. In ice each molecule is hydrogen bonded to 4 other molecules.