To answer this equation, I'll use my favorite gas equation, the ideal gas law. PV=nRT
That might look a little tricky, but I'll explain each part.
P is the pressure.
V is the volume.
n is a number related to the number of gas molecules in the container.
R is the ideal gas constant.
T is the temperature.
So if you multiply P times V, you get the same as n times R times T.
Now back to the question. If you decrease the volume of the
container, something else must change to keep our equation correct. R
can't change because it is always the same. P can change. If you squish
the container, then the pressure would go up. Or you could change n. As
you make the volume smaller, the gas just goes away into the air around
your container. You can even make T smaller. So the gas gets colder if
you keep the pressure constant while you decrease the volume.
In the end, what happens depends on what you prevent from changing. But PV=nRT will always be true.
(republished on 07/18/06)