medium difficulty6 min

Vapour Pressure, Ideal Solutions, and Deviations

Vapour pressure of a solution is the pressure exerted by vapour in equilibrium with the liquid solution; addition of a non-volatile solute lowers the vapour pressure of the solvent, and liquid pairs may show ideal or non-ideal behaviour.

In one line: Lower solvent mole fraction means lower solvent vapour pressure.

Practice This Concept

Understand it

For an ideal solution, each volatile component follows Raoult's law across the full composition range. When a non-volatile solute is added, fewer solvent molecules escape from the surface, so vapour pressure decreases. Non-ideal solutions show positive or negative deviation depending on whether unlike molecular interactions are weaker or stronger than like interactions.

Example

A solution of sugar in water has lower vapour pressure than pure water because sugar is non-volatile and reduces the mole fraction of water at the surface.

Rule to remember

For a non-volatile solute: p_solution = p_solvent⁰ x_solvent.

Relative lowering of vapour pressure: (p⁰ - p)/p⁰ = x_solute for dilute ideal solutions.

Positive deviation: observed vapour pressure is higher than Raoult's law prediction.

Negative deviation: observed vapour pressure is lower than prediction.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1If pure solvent vapour pressure is 100 kPa and solvent mole fraction in solution is 0.80, then p_solution = 100 × 0.80 = 80 kPa.
2Relative lowering = (100 - 80)/100 = 0.20, equal to solute mole fraction for an ideal dilute case.

Method you can reuse

1Identify whether the solute is volatile or non-volatile.
2For non-volatile solute, use solvent mole fraction to calculate vapour pressure.
3For deviation questions, compare A-B interactions with A-A and B-B interactions, then decide whether observed vapour pressure is higher or lower than ideal.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does adding a non-volatile solute lower the vapour pressure of a solvent?

Think first, then show answer

The mole fraction of solvent decreases, so fewer solvent molecules escape into vapour phase.

Common wrong answer

Taking x_solvent as relative lowering instead of x_solute gives the opposite numerical answer.

Don't mix this up

Students often reverse the reason for positive and negative deviations. Positive deviation occurs when A-B interactions are weaker than A-A and B-B interactions; negative deviation occurs when A-B interactions are stronger.

Exam tip

For deviation questions, compare intermolecular attractions first, then predict vapour pressure and boiling point trend.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask for relative lowering calculation, graph interpretation, identification of positive or negative deviation, or reasoning based on intermolecular forces.

1
Enough for 1 mark
Vapour pressure of a solution is the pressure exerted by vapour in equilibrium with the liquid solution; addition of a non-volatile solute lowers the vapour pressure of the solvent, and liquid pairs may show ideal or non-ideal behaviour.
2
Add for 2 marks
Example: A solution of sugar in water has lower vapour pressure than pure water because sugar is non-volatile and reduces the mole fraction of water at the surface.
3
Add for 3 marks
For an ideal solution, each volatile component follows Raoult's law across the full composition range.

MCQ Quiz

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