What is the primary factor affecting the vapour pressure of a pure liquid?

The primary factor affecting the vapour pressure of a pure liquid is temperature. As the temperature increases, the kinetic energy of the liquid molecules also increases. More molecules gain sufficient energy to overcome the intermolecular forces holding them in the liquid phase and escape into the vapour phase. This leads to a higher concentration of vapour molecules above the liquid, resulting in a higher vapour pressure at equilibrium. The nature of the liquid (intermolecular forces) also plays a crucial role, but for a given liquid, temperature is the variable that changes its vapour pressure.

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

When a non-volatile solute is added to a volatile solvent, the solute particles occupy some of the surface area of the solvent. This reduces the number of solvent molecules exposed at the surface that can escape into the vapour phase. Consequently, the rate of evaporation of the solvent decreases. While the rate of condensation of solvent molecules from the vapour phase remains largely unaffected initially, a new equilibrium is established at a lower concentration of vapour molecules, leading to a lower vapour pressure compared to the pure solvent.

What defines an ideal solution in terms of vapour pressure?

An ideal solution is one that strictly obeys Raoult's Law over the entire range of concentrations and temperatures. In terms of vapour pressure, this means that the partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction and its vapour pressure in the pure state ($P_A = X_A P_A^0$). Crucially, for an ideal solution, the intermolecular forces between solute-solvent molecules are identical to the average of the intermolecular forces between solute-solute and solvent-solvent molecules. This results in no heat change and no volume change upon mixing.

How do positive and negative deviations from Raoult's Law differ at the molecular level?

Positive deviations occur when the intermolecular forces between solute and solvent molecules (A-B) are weaker than the average of the forces between pure components (A-A and B-B). This makes it easier for molecules to escape into the vapour phase, leading to a higher vapour pressure than predicted. Conversely, negative deviations occur when A-B interactions are stronger than A-A and B-B interactions. These stronger attractions make it harder for molecules to escape, resulting in a lower vapour pressure than predicted by Raoult's Law.

Can azeotropes be separated by simple fractional distillation?

No, azeotropes cannot be separated into their pure components by simple fractional distillation. An azeotrope is a special type of liquid mixture that boils at a constant temperature and distills without changing its composition. This happens because the composition of the vapour phase is exactly the same as the composition of the liquid phase at its boiling point. Since distillation relies on differences in volatility (and thus vapour phase composition), azeotropes behave like pure compounds during distillation, making their separation by this method impossible.

What is the significance of the relative lowering of vapour pressure?

The relative lowering of vapour pressure ($\frac{P^0 - P_s}{P^0}$) is a colligative property, meaning it depends only on the number of solute particles, not their identity. Its significance lies in its direct proportionality to the mole fraction of the non-volatile solute ($X_B$). This relationship allows us to determine the molar mass of an unknown non-volatile solute by experimentally measuring the lowering of vapour pressure. It's a fundamental concept linking macroscopic properties to microscopic particle count.

Vapour Pressure of Liquid Solutions

Chemistry

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Version 1Updated 24 Mar 2026

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Vapour pressure of a liquid solution is defined as the pressure exerted by the vapours of the volatile components of the solution in equilibrium with the liquid phase at a given temperature. This equilibrium is dynamic, meaning that the rate of evaporation equals the rate of condensation. For solutions containing a non-volatile solute, the vapour pressure is primarily due to the solvent. For solut…

Quick Summary

Vapour pressure is the pressure exerted by the vapour in equilibrium with its liquid phase at a given temperature. For pure liquids, it increases with temperature. When a non-volatile solute is added to a volatile solvent, the vapour pressure of the solution decreases because fewer solvent molecules are available at the surface to vaporize.

Raoult's Law quantifies this, stating that the partial vapour pressure of a volatile component in a solution is proportional to its mole fraction ( $P_A = X_A P_A^0$ ). For solutions with multiple volatile components, the total vapour pressure is the sum of their partial pressures ( $P_{total} = X_A P_A^0 + X_B P_B^0$ ).

Ideal solutions strictly obey Raoult's Law, exhibiting no heat or volume change on mixing, with similar intermolecular forces. Non-ideal solutions deviate: positive deviations show higher vapour pressure (weaker A-B interactions, \Delta H_{mixing} > 0, \Delta V_{mixing} > 0), while negative deviations show lower vapour pressure (stronger A-B interactions, \Delta H_{mixing} < 0, \Delta V_{mixing} < 0).

Significant deviations can lead to azeotropes, constant boiling mixtures that cannot be separated by fractional distillation.

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Key Concepts

Raoult's Law for Non-Volatile Solutes

When a non-volatile solute is dissolved in a volatile solvent, only the solvent contributes to the vapour…

Raoult's Law for Volatile Solutes and Total Vapour Pressure

For a solution containing two or more volatile components, each component contributes to the total vapour…

Deviations from Raoult's Law (Molecular Basis)

Real solutions often deviate from ideal behavior. Positive deviations occur when the attractive forces…

Vapour Pressure (VP) — Pressure by vapour in equilibrium with liquid.

Raoult's Law (Non-volatile solute) —

P_s = X_{solvent} P_{solvent}^0

Relative Lowering of VP —

\frac{P_{solvent}^0 - P_s}{P_{solvent}^0} = X_{solute}

Raoult's Law (Volatile solutes) —

P_A = X_A P_A^0

P_B = X_B P_B^0

Total VP —

P_{total} = P_A + P_B = X_A P_A^0 + X_B P_B^0

Ideal Solution — Obeys Raoult's Law,

\Delta H_{mixing} = 0

\Delta V_{mixing} = 0

, A-B forces similar to A-A, B-B.

Positive Deviation —

P_{total} > P_{ideal}

, A-B forces < A-A/B-B,

\Delta H_{mixing} > 0

\Delta V_{mixing} > 0

. (e.g., Ethanol + Water)

Negative Deviation —

P_{total} < P_{ideal}

, A-B forces > A-A/B-B,

\Delta H_{mixing} < 0

\Delta V_{mixing} < 0

. (e.g., Acetone + Chloroform)

Azeotropes — Constant boiling mixtures, cannot be separated by fractional distillation.

- Minimum Boiling Azeotrope: From large positive deviation. - Maximum Boiling Azeotrope: From large negative deviation.

For deviations from Raoult's Law:

Positive Deviation: People Drink Ethanol & Water (Ethanol + Water). They feel Weaker (A-B forces weaker), get Hot ( $\Delta H_{mixing} > 0$ ), and Volume Increases ( $\Delta V_{mixing} > 0$ ). Their Vapour Pressure is High.

Negative Deviation: No Drinking Acetone & Chloroform (Acetone + Chloroform). They feel Stronger (A-B forces stronger), get Cold ( $\Delta H_{mixing} < 0$ ), and Volume Decreases ( $\Delta V_{mixing} < 0$ ). Their Vapour Pressure is Low.