# Phase diagrams for fractional distillation

• Mcp
In summary: This is because the vapor is more volatile than the liquid, and as the liquid boils, more vapor is released and the liquid becomes more concentrated.

#### Mcp

https://www.chemguide.co.uk/physical/phaseeqia/idealpd.html#top
I learned about phase diagrams involving partial Vapour composition, temperature and composition of binary solutions from this website. You can find it if you scroll down to a little above the end
First it considers a binary solution of volatile liquids. The diagram involves boiling point on Y-axis and concentration of one component on X-axis varying from 100 to 0 % (and hence 0 to 100% for other component). As composition changes, so does the boiling point of the mixture. We obtain a smooth blue curve. If we consider any point on the curve and draw a horizontal line then it will intersect a pink curve and we will consider the X-coordinate of the point of intersection which will give the composition of the vapours above the liquid developed when the liquid is at its boiling point.
I don't understand that when we take the liquid at its boiling point, it forms an equilibrium with its vapour phase whose composition is given by the pink curve in the diagram. However the amount of vapour depends on how much energy is provided to the liquid as latent heat of vaporisation. If I provide sufficient heat to liquid then all of it will convert to vapour at constant temperature(boiling point), in that case the composition of the vapours will be same as liquid (but the composition of vapours is always different from that of liquid from the diagram). This implies that the composition of vapours should depend on how much of liquid vaporises, which is never considered. Is it possible that only when all the liquid is vaporised, only then the vapour composition is equal to that of liquid and that it is given by the diagram for the rest of the cases.
Please clarify if there's a misunderstanding.

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Mcp said:
If I provide sufficient heat to liquid then all of it will convert to vapour at constant temperature(boiling point)
The temperature will not be constant for a mixture. The vapor being richer in B than the liquid, the concentration of the liquid will shift towards the right, increasing the boiling point.

DrClaude said:
The temperature will not be constant for a mixture. The vapor being richer in B than the liquid, the concentration of the liquid will shift towards the right, increasing the boiling point.
So that means that for a mixture heat at the boiling point does increase the temperature apart from being used as latent heat for phase change ? If yes, then why is it so? And corresponding to a specific composition having a specific boiling point, how to tell how much of the liquid will vaporise ?
Also you are suggesting that change in temperature changes composition and boiling point. So does this imply that azeotropes can only exist at their boiling point ?

The vapor-liquid behavior of a two component system (each with its own different boiling point when pure) is very different from that for a single component system. Are you familiar with Raoult's Law?

Chestermiller said:
The vapor-liquid behavior of a two component system (each with its own different boiling point when pure) is very different from that for a single component system. Are you familiar with Raoult's Law?
Yes but not much about the behaviour of such mixtures.

Mcp said:
So that means that for a mixture heat at the boiling point does increase the temperature apart from being used as latent heat for phase change ? If yes, then why is it so?
Because the vapor phase and the liquid phase do not have the same composition.

Mcp said:
And corresponding to a specific composition having a specific boiling point, how to tell how much of the liquid will vaporise ?
I don't understand the question here.

Mcp said:
Also you are suggesting that change in temperature changes composition and boiling point. So does this imply that azeotropes can only exist at their boiling point ?
Yes. A mixture which can have such a behavior will be an azeotrope only for a specific composition. For instance, water an ethanol form an azeotrope only at 95.6% ethanol per mass. That's why alcohol can be distilled to make stronger drinks.

Mcp said:
Yes but not much about the behaviour of such mixtures.
Raoult’s quantifies the behavior of such mixtures. If you are familiar with it (and have some experience using it), then you know that at constant pressure, the phase change does not occur at constant temperature.