Scientists have developed techniques through which gases like nitrogen , oxygen, hydrogen, and helium can be converted into liquids. These techniques involve cooling of the gas to a certain temperature and then it is compressed to a very high pressure. Due to this cooling and compression the molecules of the gas come closer and the gas gets converted into a liquid. Air is a mixture of nitrogen, oxygen and other gases and can be liquefied by cooling it to about -200 degree C at normal atmospheric pressure. Under high pressure it can be liquefied at about -141 degree C.
Through this technique the air from the atmosphere is compressed to a high pressure. This air is then allowed to expand rapidly. As a result, the air gets cooled to a very low temperature. Its heat is lost due to sudden expansion. This cool air is compressed further by which it gets converted into liquid.
Liquid air is very cool. It is a mixture of liquid oxygen which boils at -183 degree C, liquid nitrogen boils at -196 degree C and liquid argon which boils at -186 degree C. It is bluish in colour and is kept in special vacuum flasks. It is mainly used in research laboratories to produce low temperatures.
Liquid hydrogen boils at -253 degree C. It is cooler than liquid air. Liquid helium is still cooler. It boils at -269 degree C. All the liquid gases should be handled with care. It they fall on your skin they may damage the body cells. If a rubber tube is inserted in liquid air it becomes as hard as a wooden stick.
