Below the boiling point, atmospheric pressure on the liquid surface is greater than the pressure that bubbles of vapor can develop in the liquid, so these bubbles are prevented from forming. Evaporation takes place only at the liquid-gas interface. But at the boiling point, the vapor pressure becomes as great as the total pressure on the liquid surface. Bubbles of vapor begin to form inside the liquid as well as at its surface, which leads to the rapid agitation that we call boiling.

What would happen to the equilibrium vapor pressure of a liquid if some nonvolatile solute molecules or ions were added? In the pure liquid water, every molecule that approaches the surface has a certain chance of escaping into the vapor phase, depending on its kinetic energy. If a nonvolatile material such as sugar is added so that one molecule in ten is sugar and not water, then only 90% of the molecules that formerly were potential escapees have a possibility of getting out of the liquid.

The average escaping tendency of water molecules from a given amount of solution is reduced, but the rate of condensation is unaffected since no sugar molecules are present in the vapor. Condensation gets ahead of vaporization, so more vapor condenses. When vapor-liquid equilibrium is established once more, we find that the equilibrium vapor pressure is only 90% as great as it was originally