Vapor pressure lowering is one of the four colligative properties of solutions, which depend on the number of solute particles rather than their identity. The other colligative properties are boiling point elevation, freezing point depression, and osmotic pressure. It occurs when a nonvolatile solute dissolves into a solvent. The solute reduces the number of solvent molecules escaping into the air, thereby lowering the vapor pressure of the solution compared to the pure solvent. ^[1-4]

Theory

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase when molecules escape from the liquid and form a vapor above it. In a closed container, this occurs when some molecules in the liquid gain sufficient energy to escape into the gas phase. At the same time, some of the gas molecules return to the liquid. When the number of molecules leaving the liquid equals the number returning, the system is in equilibrium, and the vapor pressure stays the same. ^[1-5]

When a nonvolatile solute (a substance that does not evaporate easily) is dissolved into a liquid, the solute particles take up space at the liquid’s surface. The presence of solute particles reduces the number of solvent molecules at the surface, allowing fewer of them to escape into the air. As a result, the vapor pressure of the solution is lower than the vapor pressure of the pure liquid.

The extent to which vapor pressure is lowered depends solely on the number of solute particles in the solution, not their chemical nature, which makes it a colligative property.

Formula

Raoult’s Law explains vapor pressure lowering. It states that the vapor pressure of a solution depends on the mole fraction of the solvent and the vapor pressure of the pure solvent. The relationship can be written as: ^[1-5]

P_solution = χ_solvent x P_solvent​

Where:

– P_solution​: Vapor pressure of the solution

– χ_solvent​: Mole fraction of the solvent

– P_solvent: Vapor pressure of the pure solvent

The mole fraction (χ_solvent​​) is a way of expressing the ratio of solvent molecules to the total number of molecules (solute and solvent) in the solution. It is calculated as:

χ_solvent​ = moles of solvent / total moles of solute and solvent

This fraction indicates the proportion of the solvent in the mixture and is directly used in Raoult’s Law to determine the vapor pressure of the solution.

Raoult’s Law demonstrates that increasing the solute concentration lowers the vapor pressure, as fewer solvent molecules are available to evaporate.

The drop in vapor pressure (ΔP) can be calculated by comparing the vapor pressure of the pure solvent (P_solvent ) and the vapor pressure of the solution (P_solution​).

ΔP = P_solvent – P_solution​

Applications ^[1-5]

1. Cooking at high altitudes

At high altitudes, where atmospheric pressure is lower and water boils at a lower temperature, cooking can take longer or be less effective. Adding solutes like salt reduces water’s vapor pressure and raises its boiling point, helping to improve cooking efficiency at these altitudes.

2. Antifreeze

Ethylene glycol, a common antifreeze, lowers water’s vapor pressure, thereby reducing its freezing point and increasing its boiling point. It prevents water in car radiators from freezing in winter and boiling in summer.

3. Food preservation

Adding salt or sugar to foods reduces the vapor pressure of water in the solution, slowing evaporation. It creates an environment less conducive to bacterial growth, prolonging the food’s shelf life.

References

1. Chem.libretexts.org
2. Chem.purdue.edu
3. Jove.com 
4. Chem.uiuc.edu
5. Courses.lumenlearning.com