Question

For a pure substance, the liquid and gaseous phases can only coexist for a single value of the pressure at a given temperature. Is this also the case for an ideal solution of two volatile liquids?

Answer 1

Answer:

No, it is not.

Explanation:

Most solutions do not behave ideally. Designating two volatile  substances as A and B, we can consider the following two cases:

Case 1: If the intermolecular forces between A and B molecules are weaker than  those between A molecules and between B molecules, then there is a greater tendency  for these molecules to leave the solution than in the case of an ideal solution. Consequently,  the vapor pressure of the solution is greater than the sum of the vapor  pressures as predicted by Raoult’s law for the same concentration. This behavior gives  rise to the positive deviation.

Case 2: If A molecules attract B molecules more strongly than they do their own  kind, the vapor pressure of the solution is less than the sum of the vapor pressures as  predicted by Raoult’s law. Here we have a negative deviation.

The benzene/toluene system is an exception, since that solution behaves ideally.

Answer 2

No, in the case of an ideal solution of two volatile liquids the two substances at different state cannot coexist for a single value of pressure at a given temperature.

• The ideal solution of the two volatile liquids can exist on different ranges of pressure.
• Their pressure can be limited to an extent at which either only a trace value of liquid remains and the pressure at which only a trace value of gas exists.

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