# Confused on phase diagrams for binary systems (thermodynamics)

1. ### fluidistic

3,313
I'm confused regarding the phase diagrams of binary systems. Let's suppose we mix 2 liquids, A and B at a given pressure P that won't change during the whole experiment. I raise the temperature until I observe that some of the mix of liquids vaporizes. What boggles me is that it seems I can actually raise the temperature and there's still liquid remaining (less and less if I raise the temperature more and more), until when I raise the temperature enough and it vaporizes entirely.
But as far I as thought previously, you could not raise the temperature without the liquid passing entirely to gas if you saw it started to vaporize.
I mean, for a pure substance it seems that at a given pressure there's a single temperature at wich there's a phase transition.
Now for a mix of substance, there is no more a single temperature for which there's a phase transition. Is this correct?
If it's correct, is the latent heat meaningless for mix of substances?

### Staff: Mentor

Yes. For a solution of two components, if you hold the pressure constant, the temperature will still change depending on how much is vaporized. You can verify this by applying the phase rule. I'm not sure if the term latent heat is totally meaningless. If you carry out a flash calculation for a flow system at constant pressure, and specify the fraction of the original liquid that vaporizes, you can precisely determine the amount of heat required by doing an enthalpy balance.