Why is isothermal expansion reversible?
Isothermal expansion can surely be irreversible if you make it so. In fact, any process is usually at least a bit irreversible, and truly 100% reversible processes are impossible.
We can construct an irreversible isothermal expansion to disprove the premise of the question (which states that all isothermal expansions are reversible).
Consider the Joule expansion of an ideal gas into a vacuum, which is thermally isolated (
You clearly cannot undo expansion into a system that had 0 pressure if you only have a piston pushing on one side. Therefore, it is irreversible, as stated first thing in the article.
A reversible isothermal expansion is an infinitisimally slow increase in the volume of the substance at constant temperature.
For an ideal gas, the internal energy