Water, water everywhere but quirks still stump scientists

 

Liquid water is one of our planet's most precious commodities. Water, essential for life, covers more than two-thirds of the globe's surface and plays an important part in climate regulation. Despite our close familiarity with it, science still cannot explain some of the common behaviour of this vital fluid.

Humankind has always recognised water's importance. In the fifth century BC, the Greek philosopher Empedocles included water as one of the four basic elements that make up the world.

Apart from earth, the other planets in our solar system are generally either too hot or too cold to harbour liquid water. There is evidence, however, that liquid water may exist beneath the ice on Europa, a moon of Jupiter.

Water is not uncommon in the universe but usually exists either as a gas or as solid ice. Indeed, bulk liquid of any kind may not be common in the universe. Lakes of hydrocarbon may exist on Titan, a moon of Saturn. Liquid hydrogen and helium may also lie deep within Jupiter and Saturn, maintained in the liquid state by high pressure. Planet Earth is highly atypical, with its huge reservoir of liquid water.

Life began in the oceans of the early Earth. Water constitutes the bulk of living organisms, about 75 per cent of the mass, and the myriad biochemical reactions that constitute life all take place in water. The liquid is very versatile, with many unusual properties to which life has adapted and on which it depends.

Water is a compound of the elements hydrogen (H) and oxygen (O). The smallest part of water that can exist is the molecule, containing two hydrogen atoms and an oxygen atom - H2O. Each H atom is separately attached to the O atom, giving the molecule a Vshape with the O atom at the point of the V. Although each H 2O molecule is electrically neutral, the molecule is polarised because each O atom bears a tiny negative charge and each H atom bears a tiny positive charge.

In the liquid state the molecules are free to slide over each other but they are also attracted to each other because of the tiny opposite charges on the Hs and on the Os, that is a H on one H2O molecule is attracted to an O on an adjacent H 2O molecule. These attractions are called hydrogen bonds and explain some of water's more unusual properties.

Because it is a polarised molecule, water can dissolve a variety of compounds, an important property for the background that must support the chemistry of life. Hydrogen bonds give water a high degree of internal cohesion. This explains why water has a higher melting point, boiling point and heat of vaporisation than most other common liquids. Heat of vaporisation is the energy required to turn a unit mass of water into water vapour.

Hydrogen bonds also explain another highly unusual property of water: unlike other substances, water does not expand as it is warmed from solid to liquid, it contracts. Therefore ice is less dense than water and icebergs float. This is an important protective mechanism for aquatic life. Lakes freeze from the top down, not from the bottom up. The ice-cover insulates the water underneath from the cold air and, except under most extreme conditions, most of the water underneath remains liquid, continuing to support aquatic life.

Ice is less dense than water because, when water freezes at 0 Celsius, hydrogen bonds between the water molecules persuade them to array themselves into a regular three-dimensional lattice (like serried ranks of soldiers standing to attention) with significant spaces between adjacent molecules. As the ice melts the molecules become free to move closer to each other and, overall, they occupy less space than in the crystalline solid state.

So water density increases when the ice melts and density continues to increase up to a maximum at 4 Celsius beyond which density decreases again. This explains why the temperature of the water at the ocean bottom, where the densest water sinks, is between 2 and 4 Celsius.

The high heat of vaporisation of water is useful to living organisms because water acts as a "heat buffer", permitting the temperature of an organism to remain steady as the air's temperature changes. Also the heat of vaporisation of water is exploited by many animals as a means of losing excess body heat by evaporation of sweat.

Another marked property of water is its high surface tension. This can be looked on as a sort of invisible skin covering the surface of the water and some biological organisms exploit it to their own advantage. For example, the water stride insect which lives on pond surfaces has special hairs on its legs that enable it to rest on the water without penetrating the surface. The middle pair of legs can penetrate the surface and acts like a pair of oars which the insect uses to scull itself along.

Plants also exploit the high internal cohesion of water in transpiration - the process whereby water is absorbed by roots and transported up the stem to the leaves from where it evaporates. In some trees, the tallest leaves are 100 metres above ground. The water travels up through narrow woody tubes, the water column in each tube remaining unbroken because of the cohesion conferred by hydrogen bonds.

It has been noted for centuries that flowing water forms random swirls. Leonardo da Vinci was intrigued by the disorderly eddies in moving water and he often drew them. Analytical hydrodynamics has attempted to give a complete mathematical description of moving fluids. The resulting formula, the Navier-Stokes equation, is extremely difficult to solve.

And no one yet has been able to describe mathematically why smoothly flowing water breaks into random swirls. If physics still cannot explain such ordinary behaviour, one can easily imagine the infinity of phenomena awaiting explanation by science in the universe at large.

William Reville is a senior lecturer in biochemistry and director of microscopy at UCC