At normal atmospheric pressure, water boils at 100C. As mountaineers know, at high altitudes - and correspondingly lower pressures - water does not have to be heated as much to reach boiling point. So the temperature at which water changes from liquid to vapour or gas increases steadily with increasing pressure - but the relationship holds only up to a point.
At high temperature and pressure, the clear distinction between gas and liquid disappears. The material, then known as a supercritical fluid, becomes a hybrid, with some properties of a liquid but others that are gas-like.
The supercritical state, which for water is beyond 374C and 218 atmospheres pressure, endows water with many seemingly bizarre characteristics compared with the liquid or vapour, and it is these that chemists are trying to harness.
The most useful is its ability to dissolve almost anything, for example oils and hydrocarbons that normally separate out in water. This makes it a unique medium for chemical reactions, bringing together substances that do not normally dissolve in the same liquid; ideal for the complex brew of chemicals that make up most industrial wastes.
The aqueous fluid is also highly reactive. Dissolved oxygen in the water is often all that is needed to trigger a chain of reactions that may not even be possible with an oxidising agent many times more powerful than bleach and a metal or acid catalyst. This super-reactivity can reduce even the largest complex organic molecule to carbon dioxide and a few other simple species.
Dr Anthony Clifford, in the chemistry department at the University of Leeds, has been researching the properties of supercritical water, focusing on the speed of the reactions. 'The reactions occur extremely quickly, up to 100 times faster than ordinary chemical reactions,' he says.
The aim of his team's work is to design a practical system for the safe destruction of a broad spectrum of toxic waste. Dr Clifford is confident that this can be achieved - although it may take 10 years to accomplish because understanding of the chemistry that takes place in the medium is still hazy. 'We are still collecting basic information,' he says. 'The reactions are not understood well enough to do them on a large scale.'
Dr Clifford's work is part of a collaboration with the chemical engineering department of Imperial College, London, and the National Engineering Laboratory at East Kilbride in Scotland.
The medium shows promise for the continuing problem of disposal of polychlorinated biphenyls, or PCBs. Their use has been restricted since the late Seventies, when their extreme toxicity and persistence in the environment was recognised.
But more than 40 years of widespread industrial use, in applications as diverse as electrical transformers and capacitors to hydraulic fluids and paints, has left a huge amount of material to destroy. Specially designed incinerators, operating at temperatures above 1,100C, are the only effective solution at present.
Supercritical water may be a less energy-intensive, environmentally preferred alternative within a few years. Chemists have shown that an aqueous solution of PCBs and oxygen in the supercritical state completely destroys the lethal chloride material in minutes, at least on a laboratory scale, leaving only small benign species such as carbon dioxide. The solutions can then be safely disposed of in rivers or the sea.
'It also has the environmental advantage that the chlorine ends up as chloride dissolved in water,' Dr Clifford says. 'And there is plenty of chloride in the sea already.'
Other supercritical fluids are used as solvents for extraction in industrial processes. The best known is carbon dioxide, which removes caffeine from green coffee beans and fat from low- fat crisps. Carbon dioxide has the huge advantage of becoming critical at a mere 31C and 71 atmospheres.
Supercritical water requires temperatures and pressures 10 times higher, but 375C is cool compared with incinerator temperatures. Special pressure vessels are needed, but the closed system means that, unlike an incinerator, there are no emissions into the atmosphere.
A drawback is that the water is highly corrosive and affects stainless- steel reactors and even normally inert materials such as gold or platinum alloys. Dr Clifford does not think these properties will act as a major deterrent to exploiting the water-based medium. 'As the technology to overcome them is available, these disadvantages become cost and convenience factors to weigh against potential advantages,' he says.
The American military has been investigating the potential for chemical destruction by supercritical water since the phenomenon was first observed in the early Seventies. The environmentally non-intrusive nature of the technology adds to its attractiveness for the chemical weapons' destruction to which the US is committed, as well as for disposing of redundant stocks of other military chemicals such as propellants and
With this aim, the Armed Services Committee of Congress recently allocated dollars 180m ( pounds 121m) of its dollars 10bn ( pounds 6.7bn) research budget to support development work.
Dr Clifford, who is a member of the panel reviewing the topic for the US Army, thinks the technology will find similar application in Britain. 'In the past the problem has been swept under the carpet,' he says. 'Weapons from the Second World War that are buried underground can be dug up and disposed of by the end of the millennium using this technology.'