Answer is: Charles’s law.
Charle's law (the temperature-volume law): the volume of a given amount of gas held at constant pressure is directly proportional to the Kelvin temperature: V₁/T₁ = V₂/T₂.
For example:
V₁(gas) = 20.0 L; initial volume.
T₁(gas) = 280 K.; initial temperature
V₂(gas) = 10.0 L; final volume
T₂(gas) = ?; final temperature.
20.0 L/ 280 K = 10.0 L / T₂.
T₂ = 140 K.
As the volume goes down, the temperature also goes down, and vice-versa.
