Renee Kreisher, age 10, of Williamsport, Pennsylvania, for her question:
What exactly is liquid air?
This laboratory wonder child is full of tricky talents and tempers. Its creation. takes us into complicated chemical processes. So let's start with a few facts that we know by heart in the everyday world. Everyday water is a runny liquid and steam is a hotter form of liquid water. With this bit of chemical know how in mind, it is no strain to figure that liquid air is a colder form of gaseous air.
It is simple to understand that liquid air is super cold ordinary air. But we may have to strain our brains to grasp the temperature range and the complicated changes that go with this simple basic idea. You might suppose that the long, wintry night of the Arctic gets cold enough to freeze the air to its liquid form. It does not. Even the colder Antarctic never chills the gaseous air to its liquid state. In fact, nature never creates liquid air anywhere on the earth. In order to change gaseous air into a runny liquid, the temperature would have to drop to minus 310 degrees Fahrenheit. This is about 340 degrees colder than weather chilly enough to freeze water.
The earth's natural weather conditions cannot do the job. It is plain that creating liquid air must be done in special laboratory conditions. Even with our modern know how and equipment, the trick is not easy. Lowering the temperature is a matter of removing heat. The more heat a substance loses, the colder it gets. This can be done by several processes. Some heat can be stolen by colder .,surroundings in a refrigerator. More heat can be removed by changing the pressure and forcing the air to contract in stages.
Filtered air is squeezed under 3,000 pounds of pressure per square inch. This is about 200 times heavier than normal surface air pressure. A tank of this compressed air gets very hot. Some of this heat is removed by surrounding currents of cool, flowing water. The air then escapes through valves into small and then larger tanks. With more room it naturally spreads out and expands. But the squeezing and cooling processes have removed lots of its:, heat. Expansion back to normal removes still more heat. The air now may be down to minus 310 degrees Fahrenheit in which case some of it will change into watery drops of liquid, The liquid air must be stored in specially insulated tanks to stop ordinary outside warmth from boiling it back again into gaseous air.
The tricky job is worth the trouble because liquid air is very useful. It contains the same elements in the mixture of ordinary gaseous air. The different elements boil and become gases again at different temperatures. As the liquid warms to a certain point, all its nitrogen boils and this separated gas can be collected. Argon and oxygen boil at two warmer paints. Most liquid air is used for separating the gases mixed together in ordin¬ary air. Some liquid air is used to create vacuums and some performs useful chores in lab experiments.
Liquid air looks as harmless as liquid water. But remember its minus 310 temperature and refuse to be fooled. A touch of it freezes your flesh beyond repair. It freezes liquid mercury as hard as a pounding hammer. It makes a rubber ball as brittle as glass. Laboratory experiments show that liquid air causes surprising changes in many substances. When solid lead is dipped into the chilly liquid it becomes elastic and as bouncy as a healthy rubber ball.