Confluence News 12 (April / May 2001)

Aqua Vitae

Life as we know it depends essentially on the chemistry of the elements carbon, nitrogen, hydrogen and oxygen as well as an appropriate temperature range. Without the ability of carbon atoms to join together into long strings, organic life would not exist.

While hydrogen and oxygen independently play their part in the structure of life, when combined together as water they make a further vital contribution. The unexpected properties of water are essential to transform dusty barren rock into vibrant life.

Water is composed of molecules made up of one atom of oxygen and two atoms of hydrogen. The familiar formula H20 is one of the simplest combinations of elements and known to all school children. But on closer examination water has very strange properties. Its near neighbour, chemically, is not liquid but a gas (at room temperature). Usually the less mass a molecule has the more volatile it is, so why is such a low mass molecule not a gas? The answer is that water molecules stick together to form a mobile network, probably best imagined like an angle of wire netting but with some strands breaking and reforming continually.

All this is due to the relative sizes of oxygen and hydrogen atoms and their appetite for electrons. The large oxygen pulls electrons away from the hydrogens. The L-shaped molecule becomes negatively charged on the middle oxygen and positively charged on the two hydrogens. Opposites attract and results in the positive end of one molecule being drawn to the negative middle of another. This causes "stickiness" which holds the molecules to each other and makes water liquid between 0° and 1000° Celsius. Technically this is called hydrogen bonding and similar bonds hold DNA and proteins in shape.

But we are not finished! Water also has the property of expanding on freezing. This is again caused by the hydrogen bonds now arranging the molecules in a regular way called crystal, but taking up more space than the liquid. Such expansion splits rocks, if water has soaked into them, and eventually leads to erosion as water carries the particles away, these finally become soil or may be more rock.

Because the molecule has these negative and positive regions water also dissolves many non-carbon compounds. These substances are usually held together by minute electric charges and the water gets in the way, reducing the charge, so the solid material falls apart and 'disappears' into the water.

When water is boiled it takes in energy and becomes steam. It also expands by some 1300 times by the time steam is formed. It is this property which fuelled the steam engine and the industrial revolution.

An important and very useful property of water is that it is naturally recycled, and purified in the process. No matter how polluted the puddle, the vapour which rises from it to eventually fall as rain will be pure enough to drink!

So the next time you draw water to drink from the tap spare a thought for the millions upon millions of water molecules moving around in the glass!

Adrian Harper