Use the kinetic-molecular theory to explain the compression and expansion of gases.

According to the kinetic-molecular theory of gases, gases are composed of atoms and molecules (particles), where the distance among these elements is very large compared to their own size, therefore the total volume occupied by these particles is only a small part of the total volume occupied by the whole gas.

In other words: a gas has enough empty spaces in its total volume, therefore low density, which gives it the property of being highly compressible.

In this same sense, the kinetic-molecular theory of gases starts from the first principle of thermodynamics, establishing a relationship between heat and movement, since all material is composed of particles that are in motion (to a certain extent , depending on the state of matter), and in the case of gases the movement is greater, which is a strong indication of heat (thermal energy).

Now, gases can change their volume in two ways:

-By a change in temperature (heat transfer).

-By a change of pressure

So, this change or variation in volume is related to the work the gas does to change from an initial volume to a final volume.

Then, if in this process the volume decreases, it is said that the gas has been compressed (compression work); but if, on the contrary, the volume increases, the gas will have expanded (expansion work).

In this way, with the increase of the temperature of the gas, it is expanded by the increase of the kinetic energy of its molecules, but if an external pressure is applied it is compressed.