Λcylindcr fitted with a movable piston contains liquid water...

$\Lambda$ cylindcr fitted with a movable piston contains liquid water in cquilibrium with water vapour at $25^{\circ} \mathrm{C}$. Which opcration results in a decrcase in the cquilibrium vapour pressure? (1) Moving the piston downward a short distancc (2) Removing a small amount of vapour (3) Removing a small amount of the liquid (4) Dissolving the salt in water

$\Lambda$ cylindcr fitted with a movable piston contains liquid water in cquilibrium with water vapour at $25^{\circ} \mathrm{C}$. Which opcration results in a decrcase in the cquilibrium vapour pressure?
(1) Moving the piston downward a short distancc
(2) Removing a small amount of vapour
(3) Removing a small amount of the liquid
(4) Dissolving the salt in water

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Key Concepts

Equilibrium Vapor Pressure

This is the pressure exerted by the vapor when a liquid (or solid) is in thermodynamic equilibrium with its vapor phase at a fixed temperature. In a closed system at equilibrium, the rate of evaporation equals the rate of condensation, and the vapor pressure becomes a characteristic property of the substance at that specific temperature.

Temperature Dependence of Vapor Pressure

For a pure substance, the equilibrium vapor pressure is determined solely by its temperature. Changes in volume, pressure, or the removal of vapor or liquid (without altering temperature or composition) do not inherently change the equilibrium vapor pressure, because it is a reflection of the balance between evaporation and condensation rates at that temperature.

Raoult's Law and Vapor Pressure Lowering

When a nonvolatile solute, such as salt, is dissolved in a solvent like water, it leads to a reduction in the solvent’s vapor pressure. This is because the solute particles occupy surface positions, reducing the number of solvent molecules available to escape into the vapor phase. This effect, described by Raoult's law, is a colligative property and results in vapor pressure lowering.

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