Question

A cylinder filled with a movable piston contains liquid water in equilibrium with water vapour at ${25}^{o}C$. Which one of the following operations results in a decrease in the equilibrium vapour pressure?

• A. Moving piston downward a short distance
• B. Removing a small amount of vapour
• C. Removing a small amount of the liquid water
• D. Dissolving salt in the water

Answer 1

Answer: (D)

Dissolving salt in the water

Vapour pressure is a surface phenomenon, when the surface area is more, vapour pressure will be more. When we add solute, what we do is we decrease the vapour pressure of solvent because same of the salt ions will be present at the solvent surface, thus taking place of same of the solvent particles which were actively participating in maintain, that previous high value of vapour pressure. So, the equilibrium vapour pressure value decreases. Thus, dissolving salt in water results in decreasing of equilibrium vapour pressure.