Question

: A cylinder filled with a movable piston contains liquid water in equilibrium with water vapor at ${\text{25^\circ C}}$ .Which one of the following operations results in a decrease in the equilibrium vapor pressure?
A.Moving piston downward a short distance
B.Removing a small amount of vapor
C.Removing a small amount of the liquid water
D.Dissolving salt in the water

Answer 1

Hint: Addition of impurities in a solution always lowers the vapor pressure. Also vapor pressure of a liquid is a colligative property, i.e. it depends only on the fraction of solute and solvent present in the solution.

Complete step by step answer:
If we apply pressure at one side, then to the remaining portion pressure is transmitted within the same amount and pressure is inversely proportional to area. Vapor pressure is that the pressure exerted by the vapors over liquid. With the addition of salt in liquid water, the equilibrium vapor pressure of water will decrease because there'll be a rise within the number of solute particles and that they fill the free space present between the water. As more of the solute particles (salt) is added, there'll be less number of solvent particles (water) present on the surface and hence, the particles of solvent are going to be less to flee to the gas phase.
Therefore, this leads to the lowering of vapor pressure of liquid water when salt is added.

Hence, the write answer is option D.

Note:
Pressure is normal force per unit area. It is a scalar quantity because it's only the magnitude. SI unit of pressure is Pascal. The word vapor refers to particles of moisture that's suspended within the air, in other words, the particles of a liquid in a gaseous state. So, vapor pressure is the pressure of the vapor when in touch with its solid or liquid form.