For example, the enthalpy change required to completely vaporize \( 1 \mathrm{~mol} \) liquid water once it has reached the boiling point of \( 100^{\circ} \mathrm{C} \) at 1 bar (the molar enthalpy of vaporization) and the enthalpy change when \( 1 \mathrm{~mol} \) water vapor at \( 100^{\circ} \mathrm{C} \) condenses to liquid water at \( 100^{\circ} \mathrm{C} \) (the molar enthalpy of condensation) have the same values but opposite signs:
