Jim Hendricks, age 13, of Lynn, Mass., for his question:
HOW IS LIQUID AIR USED?
Liquid air is air so cold that it has changed from a gas to a liquid. The largest use of liquid air is in producing pure nitrogen, oxygen and other gases. Liquid air is also used industrially to cool materials and to produce s high vacuum.
Air was first liquefied in 1877 by a French physicist named Louis Cailetet. In 1883, a Polish physicist named Zygmunt Wroblewski produced the first large quantities of liquid air and 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 around 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.
When the compressed air is cool enough, it 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 air 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.
Air becomes liquid at about minus 190 degrees Celsius or minus 312 degrees Fahrenheit. Liquid air can freeze substances that do not freeze in even the coldest weather.
Liquid air is used to provide pure oxygen, nitrogen and other gases. The elements in the liquid air boil at different temperatures, so they can be separated and collected.
Liquid air is so cold that it boils when it comes into contact with warmer substances such as ice. The liquid boils because the ice is much hotter than it is.
The low temperature of liquid sir makes it behave in strange ways. When poured into a pan at room temperature, liquid air boils violently like water on a 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 leaves it before oxygen, because nitrogen has a lower boiling point than oxygen. The pale blue liquid oxygen that remains is extremely dangerous because it reacts with almost any substance that can burn. The oxygen is so concentrated in the remaining liquid that it can make a substance burn violently or even explode.