(Lansing State Journal, May 12, 1993)
Question submitted by Maggie White.
Usually when we think of oxygen, we think of the air we breathe. The air around us contains oxygen gas, as well as many other gases. If the air is cooled enough, it will change from a gas into a liquid or a solid.
Air is made mostly of oxygen and nitrogen. To get nitrogen gas changed to a liquid, it must be cooled to minus 321 degrees Fahrenheit, which is minus 196 degrees Celsius. Oxygen turns into a liquid at a slightly higher temperature, minus 183 degrees Celsius. To get these gases down to such low temperatures, the air is compressed and cooled. When it is allowed to expand again, the temperature drops even further. By repeating this process many times, the air is eventually cooled to minus 312 degrees Fahrenheit. The liquid formed is a combination of oxygen and nitrogen.
To separate the oxygen, the liquid is warmed just enough for the nitrogen to turn back into a gas - leaving only the liquid oxygen.