Betty Mather, age 14, of Atlantic City, N.J., for her question:
WHAT IS LIQUID AIR?
Liquid air is air so cold that it has changed from a gas to a liquid. Air becomes liquid at about minus 312 degrees Fahrenheit, or minus 190 degrees Centigrade. This temperature is so low that liquid air boils when it comes into contact with warmer substances such as ice. The liquid air boils because the ice is much hotter than it is.
Liquid air can freeze substances that do not freeze in even the coldest weather.
Liquid air is made of the same chemical elements as ordinary air. It is clear, slightly blue and looks like water. Unlike regular air, liquid air's elements are in a fluid state rather than gas.
The largest use of liquid air is in producing pure nitrogen, oxygen and other gases. The elements in liquid air boil at different temperatures, so they can be separated and collected. Liquid air is used industrially to cool materials. Charcoal cooled by liquid air absorbs gases well and is used to produce a high vacuum.
Air was first liquefied in 1877 by a French physicist named Louis Cailletet. In 1883, a Polish physicist named Zygmunt Wroblewski produced the first large quantities of liquid air and then in 1895 a German chemist named Carl von Linde invented the first commercial method of making liquid air.
Liquid air is now manufactured by methods based on Linde's process. Air is drawn into special factories through filters, where compressors raise the pressure to about 3,000 pounds per square inch. The air becomes hot as it is compressed.
Water jackets aroud the compressors absorb some of the heat. The compressed air is also cooled as it flows through spiral tubes inside large tanks of water. Heat from the compressed air boils the water.
The compressed air is then allowed to expand slowly and it condenses into a liquid.
The compressed air is allowed to expand through a series of valves. The air first expands into small chambers, then into larger ones. The air becomes so cold that some of it condenses into a liquid. Vapor from the cold liquid flows back around the chambers to help cool the incoming air.
Liquid air is stored in insulated tanks called Dewar flasks. These tanks are carefully insulated to prevent outside heat from boiling the liquid air. The tanks consist of one glass container inside another, similar to a Thermos bottle.
An insulating vacuum separates the containers. A silver coating on the glass may be used to reflect outside heat away from the liquid air.
When liquid air is poured into a pan at room temperature, it boils violently like water on fire. As the pan becomes cold, the air boils less violently until it just simmers.
If the pan is then placed on a cake of ice, the liquid air begins to boil again because the ice is extremely hot compared to the liquid air.
When liquid air boils, nitrogen is given off before oxygen, because nitrogen has a lower boiling point than oxygen.