# Vapor pressure of immiscible liquids

I have read that vapour pressure of a mixture of two immiscible liquids is $$P=P^0_A + P^0_B$$ where A and B are immiscible liquids.

My question is, if they are immiscible, they would form separate layers, and pressure due to the lower layer would be zero.

• Uh, what is the question? – M.A.R. ಠ_ಠ May 17 '16 at 19:14
• Will the equation be more plausible if you actually achieve a mixture by stirring the two layer system strong enough to achieve a dispersion, in which the former layers are no longer observable? – Klaus-Dieter Warzecha May 17 '16 at 19:21
• What if their densities are the same? What then is the driving force to form horizontal (rather than vertical) separation? Something else to ponder - why would a layer of B over A impact the equilibrium vapor pressure of A? (Even being 'immiscible' there is always some concentration of A in B - you can't beat the entropic contribution to the free energy). – Jon Custer May 17 '16 at 20:47