WATER

Water is important for living organisms because of it’s unusual properties.

Formula

Structure

Slightly negative at the oxygen end and slightly positive at the hydrogen end

The association between the polar water molecules

Thermal properties

The molecules of water can absorb a lot of heat energy. Water has a very high thermal capacity (4.2 J°C^-1 g^-1).The hydrogen bonding forms a lattice which does not easily fall apart as the temperature rises.

Compared to molecules of similar size and properties water has a very high melting point and boiling point.

Water is a very, thermally stable medium which helps living organisms resist changes in their environment.

To make water change from a liquid to a vapour requires a lot of energy. Evaporation of water on a the surface of a body cools it down significantly.

Solvent properties

The polar properties of water make it a good solvent for polar molecules (e.g. sugars and alcohols), which form hydrogen bonds with the water molecules, and ionic compounds (e.g. salts, acids and bases) , which dissociate into their component ions.

Water is a very important transport mediumfor living organisms because of its solvent properties and because it remains a liquid over a large range of temperatures.

Cohesion

The cohesion (stickiness) between water molecules means that a very tall thin column of water can be supported before it breaks. The tallest trees are at the physical limits of water cohesion (sequoia and eucalyptus are 100m tall).

Surface tension

Water molecules hold together forming a skin at the surface. This is strong enough for some organisms to be supported.

Density

Water is densest at 4°C whilst it is still a liquid. So ice floats on the surface of water. Organisms which live in water do not risk freezing solid so easily. Freezing is usually fatal. Water forms a good habitat for living organisms.

Transparency

Water is a transparent liquid, light passes though it. Blue light, with the most energy, penetrates furthest, red light is the weakest and penetrates least. Plants can photosynthesise under water and animals can use their visual systems.

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© Paul Billiet 2009