The element of surprise

Water, the liquid that should be a gas, is one of the most investigated yet least understood substances, says John Emsley

Water has fascinated scientists down the ages. The ancient Greek philosopher Thales thought that water was an element, and it was regarded as such until 1774, when Henry Cavendish, of Clapham, showed that it was a compound of hydrogen and oxygen. Since then it has become one of the most investigated of all chemicals, but it is still the least understood.

This month Which? magazine reported on bottled mineral waters, of which the British now buy 500 million litres a year, often paying more per litre for it than for petrol. This is odd, because most of us get cleaner and purer water from our kitchen tap, and it is a thousand times cheaper. Indeed some companies fill their bottles with tap water, and recently famous spas in France were caught using tap water to supplement their natural springs.

The annual report of the UK's Drinking Water Inspectorate shows that tap water quality is improving. Each year analysts check more than three million water samples: 99 per cent pass strict regulations governing, among other things, the levels of disease pathogens, dissolved lead, nitrates and pesticide residues. Claims by environmentalists that our tap water is a "cocktail of toxic chemicals" are simplistic nonsense.

WRc, at Marlow, Buckinghamshire, is Europe's leading water research organisation. It employs 500 people and has an annual turnover of pounds 20m. (It was previously known as the Water Research Centre, before a staff buy-out.) Dr Tony Dobbs is WRc's environmental business manager. He says that talk of increasing amounts of dangerous chemicals in water is founded on scientific ignorance. "Because water is such a good solvent, and because of remarkable advances in analytical technology, it is not surprising that we can now detect minute traces of almost anything," says Dobbs.

WRc scientists are gathering data on the quality of freshwater supplies for the European Environment Agency. Even natural spring waters contain traces of toxic materials. For example, interest is focusing on arsenic, a known carcinogen. There are relatively high levels of it in some areas and in some bottled waters. Drinking water should have fewer than 10 parts per billion of arsenic, according to the World Health Organisation.

Nothing could be simpler than a water molecule, consisting as it does of two hydrogen atoms attached to an oxygen in a V-shaped arrangement, and yet nothing is as complex in its behaviour. For example, H2O should be a gas, like hydrogen sulphide (H2S), but it is a liquid. Moreover, when it freezes at 0C its solid form, ice, floats instead of sinking. Recently, chemists have been finding water doing even stranger things.

Although water boils at 100C, this is only strictly true at sea level. At the top of Mount Everest it boils at about 75C because of the reduced air pressure. If we increase the pressure we can increase the boiling point up to 374C, but to do so requires a pressure of 220 times atmospheric. Above this, water becomes a so-called supercritical fluid, in which it behaves both like a gas and a liquid.

As such it will dissolve almost anything, even oils, and when it does, the volume of fluid can suddenly shrink to a half or less. This happens because supercritical water tends to pack tightly around other molecules. More strangely still, organic materials will flame and burn in it. Treatment with supercritical water has been suggested as an alternative to incineration for disposing of sewage sludge, which is converted to a crystal clear, odourless, germ-free solution.

When oxygen gas is pumped into supercritical water it becomes a powerful oxidising agent, able to break down some of the most persistent toxic wastes. American researchers at the Los Alamos National Laboratory in New Mexico are developing this as a way of disposing of unwanted rocket fuels, explosives and chemical weapons.

Dr Anthony Clifford of Leeds University is Britain's leading researcher into supercritical water. His group is studying the speed with which chemicals react in it. He finds that some reactions go 100 times faster than under ordinary conditions. The trouble with supercritical water is that it is capable of slowly corroding almost any metal, even gold, and the problem faced by researchers is to find a material for pressure vessels that will resist it.

The corrosive properties of water have caused enormous economic problems for the nuclear power industry. Welds inside pressurised water reactors have failed because the water has eaten away at tiny cracks, requiring huge components to be taken out and replaced, sometimes only a few years after the reactor had started operating.

Ultrasound, the frequency of which is too high for humans to hear, does remarkable things to water, creating tiny bubbles in which extremely high temperatures and pressures exist for a fraction of a second when the bubbles collapse. Under such conditions a water molecule in the bubble will cleave one of its hydrogen atoms to form the highly reactive hydroxyl (OH radical). This will then react with any other molecule it meets, and in this way dangerous or intractable materials in the water can be got rid of.

Sonochemistry, as it is called, can even eliminate the ozone-depleting chlorofluorocarbons (CFCs), which are difficult to dispose of because they were designed to be nonflammable and chemically uncreative, which is why they were widely used for 40 years in aerosols, insulation foams and cooling units. A group of Japanese chemists at Osaka University, headed by Yoshio Nagata, have demonstrated that CFCs, currently being collected from old fridges and air-conditioning units for disposal, can be converted to simple chemicals such as carbon dioxide and hydrochloric acid simply by blasting them with soundwaves in water at 20C.

Dr John Emsley is science writer in residence at Imperial College, London.