What does vapor pressure of liquids mean?

Vapor pressure

Vapor pressure, Steam tension, Pressure of the saturated steam, i.e. the steam that is in equilibrium with its liquid or solid phase in a closed vessel (steam). In the case of one-component substances, the vapor pressure only depends on the temperature, but not on the number of phases involved. The relationship between the vapor pressure and the temperature is given by the Vapor pressure curves in the p,TDiagram on (vapor pressure diagram). The three possible phases solid, liquid and vapor or gaseous are separated by three branches of a curve. Only on these branches are two phases in equilibrium possible. The boundary curve between liquid and gaseous indicates the vapor pressure of the liquid and thus also the pressure required to liquefy the vapor (or the boiling temperature as a function of the pressure). It ends at the critical point. The boundary curve between solid and gaseous indicates the vapor pressure of the solid substance, the sublimation pressure, and thus the pressure necessary for solidification (or the sublimation temperature depending on the pressure). The limit curve between liquid and solid, the melting curve, indicates the dependence of the melting temperature on the pressure. All three phases can exist simultaneously in equilibrium only at the intersection of the curves, at the triple point.

For the vapor pressure of a liquid, the equation, which has been well confirmed experimentally, follows from the Clausius-Clapeyron equation

, in which T the temperature, A. and B. Substance constants are. More precise equations can be obtained if one A. as a function of temperature.

The vapor pressure over solutions and mixtures depends on the concentration of the solution or the composition of the mixture. It is made up of the partial pressures of the components. According to Raoult's law, the partial pressures depend linearly on the concentrations (Raoult's straight line). Compared to the liquid phase, the gas phase is richer in the more volatile component. This is what makes the mixture of substances separable by distillation.

In real mixtures, deviations from the ideal behavior according to Raoult's law occur. If the interactions between the different types of particles are smaller than in the pure substances, the partial pressure curves and the curve of the total pressure show an upward curvature, so the volatility in the mixture is increased. In the opposite case, the situation is reversed. If the curve of the total vapor pressure shows a maximum or a minimum, the liquid phase and the gas phase have the same composition at the respective extreme (azeotropic mixture, Azeotrope).

The vapor pressure of very small droplets is slightly higher than in systems with a flat surface, e.g. water droplets with a radius of 10 - 5cm by 1%. The dependence of the vapor pressure of small droplets on the radius gives the Thomson's equation suggests that small droplets are not stable compared to larger ones.