James King-Turner, of Milford Sound, asks :-

A small-aircraft crash reportedly narrowly missed a container of "highly flamable liquid oxygen". Is it correct to describe oxygen as flammable?

Barrie Peake, an environmental chemist at the University of Otago, responded.

No, it is not correct to describe oxygen as flammable. Indeed this is recognised in the safety information on labels put on all vessels used to store oxygen in either gaseous or liquid form which clearly indicate that it is a ‘non-flammable’ material. Where the confusion probably lies is that there are many substances classified as flammable because they can be burnt (combusted) by reacting with typically gaseous oxygen in air to form oxides accompanied by the release of often enormous amounts of energy as heat and light (flames). So it is the oxygen either in gaseous or highly concentrated liquid form which promotes the flammability of many other compounds with which it comes in contact, rather than the oxygen burning itself.

The air we breath contains about 80 per cent nitrogen and 20 per cent oxygen as gases. By cooling air down, it is possible to liquefy both these gases and in the case of oxygen, this occurs at around -183 C. The resulting liquid oxygen is blue in colour and is around 860 times more concentrated than gaseous oxygen. Therefore much greater quantities of oxygen can be stored and more easily transported in this liquid form compared to a gas but it does require the temperature to be kept low, typically using a thermos flask-like storage tank.

So the report of the imminent danger of the aircraft crashing so close to liquid oxygen under pressure in tanks in a nearby storage facility could have arisen in at least two ways: either the crash would have destroyed the thermal insulation around the liquid oxygen tanks leading to a sudden vaporisation of the liquid oxygen which would constitute an explosion; and/or the crash would have brought already burning aircraft fuel, oil and other combustible materials in contact with the liquid oxygen causing them to burn even more violently.