Vapor pressure

Representation of steam pressure showing evaporation and condensation on the surface of the liquid.

Water vapor pressure chart.

The vapor pressure is the pressure exerted by the gas or vapor phase on the liquid phase in a closed system at a given temperature, when the liquid and vapor phases are in dynamic equilibrium. Its value is independent of the amounts of liquid and vapor present as long as both exist. This phenomenon is also presented by solids; When a solid changes to the gaseous state without going through the liquid state (a process called sublimation or the opposite process, called reverse sublimation or deposition) we also talk about vapor pressure. In equilibrium, the phases are called saturated liquid and saturated vapor. This property has a directly proportional relationship with the molecular forces, because the greater the module of the same, the greater the amount of energy delivered (whether in the form of heat or another manifestation) must be to overcome them and produce the change of state.

Initially only evaporation occurs, since there is no steam; however, as the amount of steam increases, and therefore the pressure inside the bulb, the rate of condensation also increases, until after a certain time both rates are equal. At this point, the maximum possible pressure in the bulb (saturation or vapor pressure) will have been reached: the total pressure of the gas volume (steam-air mixture) is equivalent to the partial pressure of the vapor phase (saturation pressure).. This saturation pressure can only be overcome by adding more energy (temperature) to the mixture, an action that would increase the vapor pressure (the evaporation rate), and in turn, the total pressure of the mixture (since it is a closed container).

Dynamic equilibrium will be reached more quickly the greater the contact surface between the liquid and the vapor, since this favors the evaporation of the liquid; in the same way that a large but shallow puddle of water dries up faster than a smaller but deeper one that contains the same amount of water. However, equilibrium is reached in both cases for equal pressure.

The most important factor that determines the value of the saturation pressure is the nature of the liquid, finding that, in general, among liquids of a similar nature, the vapor pressure at a given temperature is lower the higher the molecular mass of the liquid.

For example, air at sea level saturated with water vapor at 20 °C has a partial pressure of 23 mbar of water and about 780 mbar of nitrogen, 210 mbar of oxygen and 9 mbar of argon.

Measurement and units

Vapor pressure is measured in standard units of pressure. The International System of Units (SI) recognizes pressure as a unit derived from the force exerted through a given area; this unit is known by the name of pascal (Pa). One pascal is equivalent to one newton per square meter (N·m^-2 or kg·m^-1·s^-2).

Experimental measurement of vapor pressure is a simple procedure for similar pressures between 1 and 200 kPa. More accurate results are obtained near the boiling point of each particular substance and with a more significant error rate in measurements less than 1 kPa. Frequently, some procedures consist of purifying the substances being analyzed, isolating the desired substance in a container, avoiding any unwanted gases, and measuring the equilibrium pressure of the gas phase of the substance in the closed system at various temperatures. The use of tools, such as an isoteniscope, generates greater accuracy in the process.

Vapor pressure and boiling point of a liquid

A liquid is, at any temperature, in equilibrium with its own vapor when its molecules are present in a certain concentration. In this case we speak of equilibrium when saturation conditions are reached (evaporation is equaled with condensation). The pressure that corresponds to this concentration of gaseous molecules is called the vapor pressure of the liquid at the given temperature, and is a direct relationship between the partial pressure of the vapor phase (vapor pressure), and the total pressure of the vapor phase (where the evaporated component exists, and, in general, the component that previously occupied the volume, air). Therefore, knowing the vapor pressure of a liquid at a certain temperature, we can know what vapor concentration we will obtain in air under saturated conditions: water, at 20 °C, has approximately a vapor pressure of 23.4 mbar, which in relation to 1 bar of atmospheric pressure represents a 2.34% concentration by volume). Thus we know that when at 20 °C they indicate that there is a relative humidity of 100% (saturation conditions, maximum capacity of water vapor in air), they are informing us that 2.3% of the volume of air around us is water steam. The vapor pressure of each liquid increases with temperature (liquid molecules have more energy to overcome external pressure). Continuing with the example, in tropical conditions (40 °C) a humidity of 100% implies a much greater amount of water (vapor pressure of 73.8 mbar, equivalent to 7.38% of vapor in air), a fact that explains It's such a stifling environment.

The temperature for which the vapor pressure of a liquid equals the external pressure is called the boiling point of the liquid, assimilated to the phase change. At this temperature bubbles of vapor appear in the liquid and escape from the surface. For example, in a pot of boiling water you can see that bubbles appear at the bottom of the pot, where 100°C is reached faster.

Vapor pressure variation with temperature

Steam pressure versus temperature.

As a general trend, the vapor pressure of liquids at atmospheric pressure decreases with increasing boiling point. This phenomenon is illustrated in the attached diagram, which shows, for various liquids, the behavior of their vapor pressure versus temperature. For example, at any temperature, chloromethane (methyl chloride) has the highest vapor pressure of any liquid on the graph. The low boiling temperature of propane is also observed, whose vapor pressure curve (cyan line) intersects the horizontal line corresponding to 1 atmosphere at -41 °C.

Although the relationship between vapor pressure and temperature is not linear, the graph uses a vertical logarithmic axis to obtain a smooth line so that the behavior of various liquids can be represented on a single graph.

Importance for environmental law

The hazard index (Ip) of a substance is determined by the quotient between the vapor pressure of the substance and its CMP (maximum allowable concentration) under standard conditions (25 °C and 1 atm), so this Property allows us to analyze the feasibility of using a substance for certain activities, since it indicates the probability that it will volatilize.

Meaning in meteorology

In meteorology, the term vapor pressure is used to refer to the partial pressure of water vapor in the atmosphere, even if not in equilibrium, and the "pressure of balance steam" otherwise specified. Meteorologists also use the term "saturation vapor pressure" to refer to the equilibrium vapor pressure of water or brine on a flat surface, to distinguish it from equilibrium vapor pressure, which takes into account the shape and size of water droplets and particles in the atmosphere.