When two moles of hydrogen atoms join together to form a mole of hydrogen molecules in a closed rigid vessel with diathermic walls.

$\mathrm{H}\left(\mathrm{g}\right)+\mathrm{H}\left(\mathrm{g}\right)⟶{\mathrm{H}}_2\left(\mathrm{g}\right)$

In figure, $\mathrm{A}$ and $\mathrm{B}$ are two adiabatic curves for two different gases. Then $\mathrm{A}$ and $\mathrm{B}$ corresponds to:

${\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

This phase transition is carried out at constant temp and pressure then work done during the process: