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Have you ever peered into your glass of chilled water on a hot summers afternoon and wondered why the ice cubes are floating on the surface and haven’t sunk to the bottom? It is a common question and one that carries more significance than you may at first think.

For any material to sink in a liquid it has to have a higher density than the liquid it is trying to displace thereby passing through it to sink to the bottom (such as a rock sinks to the bottom of a pond when you throw it in). Water is one of the very few substances in the natural world that is actually less dense in its solid form (ice) allowing it to float.

But why does water have this unique property? Well to understand that we have to get our heads around a little chemistry.

A water molecule is made up of two hydrogen atoms and one oxygen atom (hence H2O). The bonds that hold these atoms together are extremely strong and are known as covalent bonds. A covalent bond is where two atoms share electrons between them often to form one link in a molecule. But there is another weaker chemical bond at play in water that gives it its unique property and these are known as hydrogen bonds.

Every water molecule is bent in the sense that the two hydrogen atoms tend to stick out at angles giving the molecule a roughly ‘L’ shape. This has the effect of making the molecule polar, which translated means that one side (the hydrogen atoms) is positively charged whilst the other side (the oxygen atom) is negatively charged. The hydrogen bonds form when positively charged hydrogen atoms are attracted to negatively charged oxygen atoms.

In a liquid state the hydrogen bonds in water are weak and short lived as the molecules contain lots of energy meaning they are constantly moving about. As water cools towards 4C this frenetic activity slows down until the molecules begin to ‘keep’ their hydrogen bonds, locking into a crystalline lattice with their neighbours. Crucially it just so happens that in this solid lattice like state the water molecules are actually set further away from each other meaning that their density is lower (ie there are less water molecules in any given volume of space).

And so therefore the density of water in its solid state (the ice cubes in your glass) is lower than the density of water in its liquid state (the water in your glass) allowing your ice cubes to bob happily on the surface of your drink.

Without the ability to float on water, ice forming on the surface of seas, lakes and rivers would sink straight to the bottom, thereby failing to insulate the exposed surface water from the cold temperatures above resulting in the whole body of water slowly freezing solid. As all life on earth originated in liquid water our single celled ancestors wouldn’t have even got going and the earth would have turned out to be a very lonely and desolate place from the one we inhabit now.

© 2012 Office Water Coolers

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