The vapor pressure is the equilibrium partial pressure over a condensed phase. The vapor pressure mainly depends on the volatility of the compound and temperature. The vapor pressure curve of a pure substance, which begins at the triple point and ends at the critical point, can be described using Clausius-Clapeyron equation . With a solute present, the vapor pressure is lower than with the pure solvent. Vapor pressure reduction is proportional to the concentration of the solute and described by Raoult s 1st law. When we dissolve a gas in a liquid, the amount of gas dissolved is proportional to the partial pressure of the gas above the liquid - that's Henry's law.