When solute particles are added to a pure solvent in a closed container at constant temperature and pressure, the vapor pressure of the solvent decreases. This phenomenon is known as Raoult's Law.

Here's why:

* Vapor Pressure: The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in equilibrium.

* Solute-Solvent Interactions: When a solute is added to a solvent, the solute particles interact with the solvent molecules. These interactions can be stronger or weaker than the solvent-solvent interactions.

* Reduced Solvent Surface Area: The presence of solute particles reduces the surface area of the solvent exposed to the vapor phase.

* Lower Escape Rate: With a reduced surface area and stronger interactions, the solvent molecules have a lower chance of escaping into the vapor phase, resulting in a lower vapor pressure.

Raoult's Law states that the partial vapor pressure of a solvent in a solution is equal to the vapor pressure of the pure solvent multiplied by the mole fraction of the solvent in the solution:

* P_solution = X_solvent * P_solvent

where:

* P_solution is the vapor pressure of the solution

* X_solvent is the mole fraction of the solvent in the solution

* P_solvent is the vapor pressure of the pure solvent

Important Note: This explanation assumes an ideal solution, where the solute and solvent molecules interact similarly to each other. In real-world scenarios, deviations from Raoult's Law may occur.