Charles's law
/ SHARLZ /
Warm a gas at constant pressure and it expands; the hotter it gets, the more room it takes up. This is how a hot-air balloon rises: heating the air inside makes it expand, so it becomes less dense than the cooler air outside and floats upward.
Precisely: at constant pressure and a fixed amount of gas, volume is directly proportional to absolute temperature, V / T = constant, or V1/T1 = V2/T2, with T in kelvin. It was found by Jacques Charles around 1787.
This law points straight to absolute zero: extrapolating the volume down toward V approaching 0 corresponds to -273.15 degrees C, which is where the idea of absolute zero comes from. Remember that T must be in kelvin, not Celsius, for the proportion to hold.
Put an inflated balloon in the freezer and it visibly shrinks; bring it back to room temperature and it swells again.
At fixed pressure, V is proportional to absolute temperature.
The volume is proportional to the kelvin temperature, not the Celsius one: doubling from 10 to 20 degrees C does not double the volume, but doubling from 150 K to 300 K does.