Consider a liquid in a piston apparatus that is at equilibrium with the vapor phase. in which direction should the piston be moved to decrease the number of molecules in the liquid phase?

the piston should be in the upmost position to allow room for material to expand, thus be in vapor state, when a gas is compressed, it liquefies, and when dealing with a piston, much like in an auto combustion engine, the piston moves up and allows oxygen to mix with vapor and explode, thus powering further movement of pistons.