QUESTION #454

# Why does air get colder as it expands?

The frequency of atomic collisions decrease as air expands, therefore the air gets cooler.

Temperature is just the average heat of a substance. That is, if you take the kinetic energy (heat) of all the particles in a given volume, and divide by the volume you get the energy density, which we call temperature.

If you have a certain amount of air, the particles have a fixed amount of heat (unless you let the heat pass to some other substance or use some to do work) and so if you let the air expand you decrease the temperature (mathematically you are dividing the heat by a larger number). Thus the blast of a CO2 fire extinguisher can be used to cool a can of your favorite beverage, or freeze an attacking Blob! (for you monster movie buffs out there!)
Answered by: Rob Landolfi, Science Teacher, Washington, DC

The Ideal Gas Law states pV = nRT, where P = The pressure of the gas in Pa, V = Volume of gas in m3, n = Number of moles of gas, R = A constant of about 8.314 and T = Temperature in K.

As a gas (like air) expands, the value of V increases and this has the effect of increasing T (The temperature). As the energy needed to increase it's temperature must be supplied from somewhere, the gas takes the energy from the surrounding system giving the effect of cooling. This is a principle used in refrigeration.