Hi I have difficuly understqanding this: If a gas undergoes changes in its thermodynamic states, then the process is irrversible unless it is conducted at such an infinitely slow pace that at every instant of the process, the system is in thermodynamic equilibrium. Ok, so the text gives the example of a gas expanding into a vacuum without heat transfer like a adiabatic process. The problem is: The text says that the expanding gas does no work and after its expansion, we cannot reverse it because compressing the gas means that work is removed by the system from the surroundings resulting in a rise in temperature which cannot be dissipated as work completely according to the second law of thermodynamics. I agree with the text. Except that what if the gas was expanding on its own uin a non quasi static manner? It would have done work against the surroundings, the temperature of the gas would have decreased and I can reverse all this by supplying the work done on me by compressing the gas and the gas's temperature would have risen to where it was initially. This is where I do not see how a quasi static process is essential to ensure that the provess is reversible.