Vapor Pressure Lowering

Some properties of a solution depend on the type of solute. An aqueous solution of hydrochloric acid turns a pH paper red, while a sodium hydroxide solution turns it blue.

Other properties of a solution depend only on the concentration or the number of particles of the solute rather than the type of solute. These are called colligative properties.

One such property is the vapor pressure of a solution. The vapor pressure of a liquid is the pressure of the gas above the liquid resulting from evaporation when the liquid and the gas are in a dynamic equilibrium in a closed container.

The vapor pressure of the solution is always less than that of the pure solvent.

Consider a solution made when a non-volatile solute, that is one with no measurable vapor pressure, is added to a volatile solvent.

In a pure solvent, the entire surface of the liquid is solvent particles. Some of these particles escape into the gaseous state to create vapor, while some of the gas molecules above condense back into a liquid state.

When the rate of vaporization equals the rate of condensation, a dynamic equilibrium is reached.

In a solution, the liquid surface has both solute and solvent particles. So, a smalle