Paul Cairns, age 11, of Stratford, Conn. for his question:

What is liquid oxygen?

Of all the elements in our world, oxygen is the most important. All the living plants and animals on our luxury planet depend upon oxygen in its many different forms. Luckily it is the most plentiful of all our elements. It combines with hydrogen to form water. It combines with other elements to form most of the solid rocks of the earth's crust. Gaseous oxygen makes up 21 percent of the air.

There is plenty of oxygen around as a free floating gas in the air. There is oxygen in water and in solid forms. There is oxygen in the air we breathe, in the water we drink, in the food we eat and in the solid earth on which we stand. But the chemist does not count water as the liquid form of oxygen or rocks as the solid forms of oxygen.

These liquids and solids are compounds and compounds are made from bundles of assorted atoms. A particle of liquid water contains an atom of hydrogen bound together with two atoms of oxygen. .A. particle of silca rock contains an atom of silicon bound with two atoms of oxygen. Liquid oxygen is pure oxygen chilled to its liquid state. Chilled still further, liquid oxygen reaches a frozen solid state.

We are used to the idea that chilly water freezes solid at 0 centigrade degrees. At 100 degrees centigrade it boils and becomes gassy vapor. Free oxygen, when not combined with other elements, boils to its gaseous form at ordinary temperatures. But when it chills to minus 183 degrees it becomes a clear, bluish liquid. At the bitterly cold temperature of minus 218 degrees, liquid oxygen freezes into an icy blue solid.

What kind of a refrigerator do we use to chill oxygen gas 183 degrees colder than ordinary ice? The answer is, we use a different trick for chilling oxygen. Then a gas is compressed or squeezed, it loses heat. When the compressed gas is then allowed to escape through a tiny hole, it expands in a hurry and in so doing it loses still mare of its heat.

The oxygen gas is squeezed under a pressure of 49.7 atmospheres  which is about two and a half tons of weight crushing down on every square inch. The gassy molecules are ,jammed into a very small apace. Then the chilly substance is released through a small opening and it comes dripping out as pale bluish liquid oxygen.

Oxygen is, of course, a fuel element necessary to fire, rust and other forms of combustion. It is one of the fuels used to take our rockets up into space. Someday our rockets will be powered by nuclear energy but at present we must use chemical fuels such as liquid .