You have a sealed glass jar full of air. If you put it in the freezer, what happens to the gas pressure in the jar?

The pressure of the gas inside the jar decreases, because you have

reduced the temperature of a fixed mass and fixed volume of gas.

While the pressure inside the jar decreases, the pressure outside the

jar doesn't change. So the pressure outside becomes greater than the

pressure inside, and that's why you may see the lid of the jar sag inward,

pushed by the greater pressure on the outside.

The air will try to compress but it can't