We compare physical descriptions of gas expansion.
In contrast to a solid, a gas has no fixed structure. The gas molecules are not bound to each other but move around very freely, which is why a gas does not have a well-defined shape of its own. pdf 
When a gas expands it does work on its surroundings. That work is equal to the area under the curve on a pressure-volume diagram. post
The difference between reversible and irreversible processes is brought out through examination of the isothermal expansion of an ideal gas. The question to be asked is what is the difference between the ``free expansion'' of a gas and the isothermal expansion against a piston? post
Ideal Gas Law Equation allows you to calculate the pressure, volume, temperature, or moles of a gas.