# Does adding a nonvolatile solute to a liquid tend to increase or to decrease its vapor pressure?

May 31, 2017

It ALWAYS decreases the vapor pressure when it's a nonvolatile solute.

A nonvolatile solute does not vaporize at all. Hence, it does not contribute to the vapor pressure above the solution, and we can focus on the effect of the solute being dissolved in solution on the vapor pressure.

By Raoult's law for ideal solutions:

${P}_{A} = {\chi}_{A \left(l\right)} {P}_{A}^{\text{*}}$,

where:

• ${P}_{A}$ is the vapor pressure above the solution of solvent $A$.
• $\text{*}$ indicates pure solvent.
• ${\chi}_{A \left(l\right)}$ is the mol fraction of $A$ in the solution phase.

If we add a nonvolatile solute $B$, then we must have that ${\chi}_{A \left(l\right)} < 1$, because anything added in will lower the fraction from a maximum of $1$ to be below $1$. Therefore...

${P}_{A} < {P}_{A}^{\text{*}}$

when ANY nonvolatile solute $B$ is mixed in. This means that the vapor pressure ${P}_{A}$ of the solvent $A$ must have decreased due to adding $B$, from its pure vapor pressure ${P}_{A}^{\text{*}}$.