If you pick up a cubic centimeter of aluminum at room temperature, it will contain 6,000 billion billion atoms of aluminum, at standard pressure, at no pressure or at 10 times standard pressure. If you collect a liter of oxygen from the atmosphere at room temperature it will contain about 250 billion billion atoms of oxygen. But you could also collect a liter of oxygen pressurized at two atmospheres and it would contain 500 billion billion atoms of oxygen. That's because you can compress oxygen at just about any temperature you choose.
Oxygen behaves as an ideal gas, which means it follows a very famous equation called the ideal gas law. The ideal gas law says that if you collect one liter of oxygen at three, seven or 10 times standard atmospheric pressure, you'll get three, seven or 10 times as many oxygen atoms in that liter as you would at standard pressure. In the same way as you can fill a bicycle tire with more air by adding pressure, you can compress oxygen with any pressure your container will hold.
Oxygen, like most other elements, can exist in three forms: liquid, solid or gaseous. Under normal circumstances, you will encounter gaseous oxygen. That's because oxygen cannot be liquified above a temperature of -119 degrees Celsius (-182 degrees Fahrenheit), no matter how much you compress it. At its critical temperature, it takes a pressure of 49.2 atmospheres to liquify oxygen. As you lower the temperature below critical, you need less pressure to liquify oxygen. If you want to liquify oxygen at standard pressure, that is, 1 atmosphere, you'll need to lower the temperature to -191 degrees Celsius (-312 degrees Fahrenheit).
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