Substance is in equilibrium between its vapour and liquid

[MathewsMD]

In Chemistry, if a substance is in equilibrium between its vapour and liquid, does it account for the dissolved gas molecules?

I have always heard that the rate of evaporate = rate of condensation in a case like this, but does the rate of evaporation include the dissolved gases escaping and other gas molecules dissolving? Since can this be technically considered a change of state since it is becoming aqueous (if water solvent) but what is it called if the solvent is the same substance? In either case, water or the same substance acting as the solvent, is this accounted for in the equilibrium equation or is this completely different?

 

[Chestermiller]

In Chemistry, if a substance is in equilibrium between its vapour and liquid, does it account for the dissolved gas molecules?

I have always heard that the rate of evaporate = rate of condensation in a case like this, but does the rate of evaporation include the dissolved gases escaping and other gas molecules dissolving? Since can this be technically considered a change of state since it is becoming aqueous (if water solvent) but what is it called if the solvent is the same substance? In either case, water or the same substance acting as the solvent, is this accounted for in the equilibrium equation or is this completely different?

The short answer is that it's already accounted for. It is not possible to distinguish "dissolved" water molecules in the liquid from the rest of the molecules in the liquid. Once a water molecule enters the liquid, it is just the same as any other water molecule in the liquid (since it undergoes rapid collisions and exchanges energy rapidly).

 

[Borek]

Each substance present in the solution has its own separate gas/liquid equilibrium, so you can have solution that is in equilibrium in regard to one substance and not in equilibrium with regard to another. But it can also be in equilibrium in regard to all substances.