Cheryl Mason, age 10, of Ottawa, Ontario, Canada, for her question:
How can they turn nitrogen into a liquid?
Boiling water, changing into steamy gas, is a whole lot hotter than frozen ice. But the ice is almost twice this much hotter than liquid nitrogen. While water is boiling and freezing, free nitrogen remains an airy gas. It must be chilled and then chilled a lot more before it changes into a watery liquid.
The ingredients for creating liquid nitrogen are as plentiful as the air itself. In fact the best ingredient for the job is ordinary air, cleaned and free of most of its impurities. It is, of course, a mixture of gases and about four fifths of it is gaseous nitrogen. Almost all the rest of it is gaseous oxygen. The ingredients are cheap and plentiful but the operation of changing these gases into liquid air is very costly. They are chilled together below their boiling points and then separated as the liquid nitrogen evaporates back to its gaseous form.
The gaseous, liquid and solid states of mater are related to changes in temperature. This is because they are made of molecules and these bitsy particles of matter use warmth as energy to move around. Gas molecules have enough zip to separate, spread out and move around at terrific speeds. As a substance grows cooler, its molecules loose energy, slow down and crowd closer together. Below the boiling point they cling together in liquid form. When a substance reaches its freezing point, its crowded molecules are locked together in more or less fixed positions.
Liquid air is ordinary air chilled below the boiling point of nitrogen and oxygen to about minus 190 degrees Centigrade. This is 190 degrees colder than ordinary ice, which is only 100 degrees colder than boiling water. The job is done by compressing or squeezing the air, cooling it and then allowing it to expand or spread out again. This cycle of operations is repeated several times to bring the temperature down to the liquifying level.
The air piped into the compressor is squeezed under pressure equal to about on ton per thimbleful. This crowds the molecules closer together but it makes the gas hotter. So the compressed air is circulated through pipes coiled through a tank of cool water. This steals some of the heat. So the water cooled air is allowed to escape through tiny valve and spread out throuAk a coil of wider pipes. This cools it some more, but not yet enough. It is sent back to the compressor for another round trip through the water tank and the expansion tank. Each trip lowers the temperature until the gaseous mixture reaches minus 190 degrees Centigrade. At this temperature it begins to settle in trickling drops of liquid air. About one fifth of the mixture is liquid oxygen and the rest is liquid nitrogen.
The boiling point of nitrogen is about 13 degrees above the boiling point of oxygen. When liquid air is allowed to stand at room temperature, the nitrogen boils away at a furious rate and changes itself back into a gas. The pure nitrogen gas can be captured and returned to the liquifying machinery to be compressed, and condensed and allowed to expand until it chills to around minus 196 degrees Centigrade. At this point it changes into the liquid form of pure nitrogen.