Question

Consider a beaker of salt water sitting open in a room. Over time does the vapor pressure increase, decrease, or stay the same? Explain

Short answer

The vapour pressure of water over the solution surface will decrease

Step-by-step solution

Concept

The pressure over a solution of an in volatile salt in a volatile solvent is given by $\text{P}=\chi {\text{H}}_{\text{2}}0\text{P}°{\text{H}}_{2}0$where

$\chi {\text{H}}_{2}0$is the mole fraction of ${\text{H}}_2\text{0}$.

$\text{P}°{\text{H}}_2\text{0}$is the water vapour of pure ${\text{H}}_2\text{0}$.

Step 2:Explain the reasoning

As water evaporates, the solution becomes concentrated and the mole fraction of the water will decrease. Therefore, the vapour pressure of water over the solution surface will decrease.