# When benzene vaporizes into a vacuum at 60^@ "C" and "1 atm", is it reversible or irreversible?

May 17, 2017

Irreversible, or at least, not perfectly reversible (meaning, irreversible...).

Well, vaporization of benzene in a closed container previously containing zero gas particles (a vacuum) is, even assuming complete vaporization, irreversible.

Note that this is NOT the normal boiling conditions (the boiling point is ${80.1}^{\circ} \text{C}$ at $\text{1 atm}$), and this is just natural evaporation in the gas region of benzene's phase diagram, at ${60}^{\circ} \text{C}$ and $\text{0 atm}$:

By the time the formed gas particles have equilibrated, we have seen that:

• the initial state is no gas particles in the container, only liquid benzene.
• the final state is that gas particles were (assumed) completely converted from liquid benzene.

However, if we were to try to reverse the process and condense the benzene back into a liquid, we could not do so without letting air in, or without lowering the temperature, or some other method that is not the exact reverse of the forward process, because natural evaporation is always spontaneous for a volatile liquid in the gas region of its phase diagram.

Thus, we cannot perfectly regenerate the liquid in the vacuum, and the vaporization process into a vacuum cannot be reversed exactly.