Vapor Pressure Lowering
Definition and meaning of Vapor Pressure Lowering in chemistry.
Vapor pressure lowering is a colligative property describing how adding a nonvolatile solute to a pure solvent decreases the solvent's vapor pressure. The dissolved particles physically interfere with the solvent molecules' ability to escape into the gas phase.
In more detail
Vapor pressure lowering is a fundamental colligative property that occurs when a nonvolatile solute is dissolved into a pure liquid solvent. Vapor pressure is the outward pressure exerted by gas molecules hovering directly above a liquid as they continuously evaporate and condense. In a pure solvent, the surface of the liquid is entirely occupied by solvent molecules, allowing them to freely escape into the gas phase.
However, when a solute is added, the solute particles occupy physical space at the surface of the mixture. Because there are fewer solvent molecules at the surface exposed to the air, fewer of them can escape, leading to a noticeable drop in overall vapor pressure.
This phenomenon is mathematically modeled by Raoult's Law. Raoult's Law states that the vapor pressure of a solution is directly proportional to the mole fraction of the solvent present in that solution. If a chemist adds more solute to the mixture, the mole fraction of the solvent naturally decreases, causing the vapor pressure to drop even further.
Because vapor pressure lowering is a colligative property, the exact identity or chemical makeup of the solute does not matter at all. The only factor that dictates the severity of the pressure drop is the total number of dissolved particles. Vapor pressure lowering is the underlying cause for two other extremely important colligative properties: boiling point elevation and freezing point depression.
A liquid physically boils when its internal vapor pressure equals the surrounding atmospheric pressure. Because the added solute lowers the vapor pressure, the solution must be heated to a much higher temperature to force the vapor pressure back up to atmospheric levels, thus elevating the boiling point.
Similarly, the lowered vapor pressure disrupts the formation of a solid crystal lattice, forcing the freezing point of the solution to drop.
Key facts
| Field | Physical Chemistry |
|---|---|
| Type of Property | Colligative property |
| Cause | Solute particles blocking solvent molecules at the surface |
| Governing Rule | Raoult's Law |
| Key Dependency | Depends only on the number of particles, not their identity |
| Related Effects | Directly causes boiling point elevation and freezing point depression |
Adding table salt to a beaker of pure water lowers the water's vapor pressure, which prevents the water from evaporating as quickly as it normally would.
Frequently asked questions
What does nonvolatile mean?
A nonvolatile substance does not easily evaporate into a gas at room temperature, meaning it contributes zero vapor pressure of its own to the solution.
How do ionic compounds affect vapor pressure lowering?
Ionic compounds dissociate into multiple separate ions when dissolved, multiplying the total number of particles and causing a more drastic lowering of vapor pressure.
Does vapor pressure lowering stop evaporation completely?
No, it just significantly reduces the rate of evaporation because there are fewer opportunities for the solvent molecules to escape the liquid.