Question

A non-volatile solute……….. (increase\decrease) the rate of evaporation of the solvent and thereby decrease the vapor pressure of the solvent.

Answer 1

Hint: We need to know that a pure substance has well-defined physical properties such as freezing and boiling points. In contrast, many properties of a solution depend on the concentration of solute. Properties of solutions that depend on the concentration (but not on the identity) of solute are called colligative (collective) properties.

Complete answer:
We must remember that a non-volatile solute decreases the rate of evaporation of the solvent and thereby decreases the vapor pressure of the solvent.
If you add a nonvolatile solute to solvent such as water, you decrease the tendency for water molecules to evaporate into the gas phase. As a result, fewer molecules change from the liquid to the gas phase, thus reducing the vapor pressure. The vapor pressure of a solvent is lowered by the addition of a non-volatile solute to form a solution. This decrease in vapor pressure can be explained by using the entropy differences of the liquid and gas phases along with the position of dissolved particles after the addition of the solute. A nonvolatile solute has a vapor pressure of zero, so the vapor pressure of the solution is less than the vapor pressure of the solvent. Thus, a higher temperature is needed for the vapor pressure to reach the surrounding pressure, and the boiling point is elevated.

Note:
We need to remember that vapor pressure is the pressure inside a closed container when a liquid is in equilibrium with its own gas phase so the rate of evaporation is equal to the rate of condensation. When a non-volatile solute is dissolved in a volatile solvent, the vapor pressure of the solvent is reduced. As the concentration of the solute increases, the vapor pressure of the solvent decreases.