TEMPERATURE, VOLUME, AND PRESSURE

A sample of gas confined to a rigid container will exert more and more pressure on the walls of the container as the temperature goes up. If the container is flexible, such as a balloon, the volume of the gas will increase. Similarly, if you take a container with a certain amount of gas in it and suddenly make the container bigger without adding any more gas, the drop in pressure will produce a decrease in temperature. If you have a rigid container with gas in it and then some of the gas is allowed to escape (or is pumped out), the drop in pressure will chill the container. This is why, for example, a compressed-air canister gets cold when you use it to blow dust out of your computer keyboard.
Liquids behave a little more strangely. The volume of the liquid water in a kettle and the pressure it exerts on the kettle walls don’t change when the temperature goes up and down unless the water freezes or boils. Some liquids, however, unlike water, expand when they heat up. Mercury is an example.
This is how an old-fashioned thermometer works. Solids, in general, expand when the temperature rises and contract when the temperature falls. In many cases you don’t notice this expansion and contraction. Does your desk look bigger when the room is 30°C than it does when the room is only 20°C? Of course not. But it is! You don’t see the difference because it is microscopic. However, the bimetallic strip in the thermostat, which controls the furnace or air conditioner, bends considerably when one of its metals expands or contracts just a tiny bit more than the other. If you hold such a strip near a hot flame, you actually can watch it curl up or straighten out.