A typical free expansion scenario described in texts is that of a gas confined to a thermally insulated container with a removable membrane. When the membrane is removed the gas expands into vacuum. I understand that no work can be done because the gas does not exert a force. However, I am confused by the statement given at this source: http://www.etomica.org/app/modules/sites/JouleThomson/Background2.html Which says that because the expansion is free, the change in volume is zero. Obviously this must be the case because this is the only way for the area under the path on the PV curve to equal zero. Can anyone articulate this idea? It is not obvious to me why we can say the change in volume during free expansion is zero.