If compressing a gas makes it hot, why is liquefied gas cold? --- Jamie, Sterling, Va.
Marshall Brain Answers:
It is true that compressing a gas makes it hotter. This is why the coils on the back of a refrigerator get hot. They contain compressed gas that is in the process of condensing into a liquid. See how refrigerators work for details.
But it is not true that liquefied gases are always cold. Take the case of propane, which is a liquified gas that sits in a tank by the grill. Propane is naturally a gas at room temperature and normal sea level pressure. But if you compress propane and stick it in a tank, it will turn into a liquid. It is hot when you compress it, but that heat dissipates and the propane turns into a liquid.
A closed tank of propane sitting on a shelf will have the same temperature as the air around it. The liquefied propane is not cold. It is when you open the valve and start releasing propane that the propane liquid gets cold. When you open the valve, you lower the pressure inside the tank, to the point where the propane starts boiling. The boiling makes sense, given that propane wants to be a gas at room temperature and normal sea level pressure. But the boiling point of propane is -43 degrees F. So the liquid propane's temperature becomes very cold as it is boiling. If you close the valve, the propane's pressure in the tank will build back up. The boiling stops. The liquid propane returns to room temperature.
The moral of the story: A liquified gas is "cold" only if it is evaporating or boiling. If it is boiling, its temperature is controlled by the boiling point of the liquid. Propane boils at -43 degrees F. Nitrogen boils at -320 degrees F. And so on.