Vapor pressure

Vapor pressure is the pressure that a gas, called a vapor, puts on its liquid or solid form when they are in balance at a certain temperature. This happens in a closed space where the liquid and the gas are not changing each other. When a liquid has high vapor pressure, it means it can easily turn into a gas, and we call such liquids volatile.

As temperature goes up, more of the liquid turns into gas, which makes the vapor pressure go up too. Liquids that stick together very well, like water, have lower vapor pressure, while liquids that don’t stick as well have higher vapor pressure.

The temperature at which a liquid’s vapor pressure matches the air pressure around it is when the liquid boils. At this point, the liquid starts to form bubbles of gas. The pressure of the air and the depth of the liquid affect when these bubbles start to form. Very tiny bubbles need a little extra pressure to stay together because of their surface tension.

Measurement and units

Vapor pressure is measured using special units that tell us about pressure. The most common unit is called the pascal (Pa), which measures how much force is pushing on an area.

To find vapor pressure, scientists clean the material they are testing, put it in a container, and then measure the pressure of the gas that forms above it at different temperatures. This helps them understand how the material behaves under different conditions. Sometimes, doctors use another unit called millimeters of mercury (mmHg) to measure vapor pressure, especially for certain medicines used during surgeries.

Estimating vapor pressures with Antoine equation

The Antoine equation is a useful math formula that connects vapor pressure with temperature for pure liquids or solids. It works by fitting a curve to how vapor pressure usually goes up as temperature goes up.

The basic form looks like this:
log P = A − B / (C + T)

Here’s what each part means:

P is the vapor pressure of the substance
T is the temperature
A, B, and C are special numbers that change depending on the substance

There’s a simpler version too:
log P = A − B / T

Both equations help scientists figure out vapor pressure at different temperatures. But they work best only within certain temperature ranges, and they aren’t always super exact, especially at very low pressures.

Relation to boiling point of liquids

Further information: Boiling point

Vapor pressure is the pressure of a gas above a liquid or solid at a certain temperature. Generally, liquids with lower boiling points have higher vapor pressures at normal temperatures. This means they turn into gas more easily.

At the boiling point of a liquid, its vapor pressure matches the pressure of the air around it. For example, methyl chloride boils at a very low temperature and has a high vapor pressure, meaning it easily becomes a gas.

Liquid mixtures: Raoult's law

Raoult's law helps us understand the vapor pressure of mixtures of liquids. It tells us that the total vapor pressure of a liquid mixture depends on the vapor pressures of each liquid in the mixture and how much of each liquid is present.

When a mixture has a higher vapor pressure than Raoult's law predicts, it shows that the liquids in the mixture attract each other less strongly. An example is a mix of about 95% ethanol and water, which boils at a lower temperature than either pure liquid.

Sometimes, a mixture has a lower vapor pressure than expected. This happens when the liquids attract each other more strongly than they do in pure form. For example, a mix of trichloromethane (chloroform) and 2-propanone (acetone) boils at a higher temperature than either pure liquid.

Solids

Vapor pressure of liquid and solid benzene

When a solid and its vapor are in balance, we call this the equilibrium vapor pressure. For solids like crystals, this happens when the speed at which the solid turns into vapor (sublimation) matches the speed at which the vapor turns back into solid. Most solids have very low vapor pressure, making it hard to measure. Special tools like thermogravimetry can help find these pressures.

There are ways to figure out the vapor pressure of a solid. One way uses the vapor pressure of a liquid that has been cooled below its freezing point. This uses a special math formula that helps estimate the solid’s vapor pressure if we know how much energy is needed to turn the solid into a liquid.

Boiling point of water

Main article: Vapour pressure of water

Graph of water vapor pressure versus temperature. At the normal boiling point of 100 °C, it equals the standard atmospheric pressure of 760 torr or 101.325 kPa.

Water boils when the pressure of its vapor matches the pressure around it. This is why water boils at lower temperatures on mountains, where the air pressure is lower. There are special formulas that help us figure out the boiling temperature of water based on the air pressure, but these are more for scientists and don't need to worry us here.

Dühring's rule

Main article: Dühring's rule

Dühring's rule says that for two solutions, the temperatures at which they have the same vapor pressure are connected by a straight line. This means that if you know the temperature for one solution, you can figure out the temperature for the other solution using a simple straight-line relationship.

Examples

Here is a list of different materials arranged by how much vapor pressure they have, from the lowest to the highest.

Substance	Vapor pressure			Temperature (°C)
Substance	(Pa)	(bar)	(mmHg)	Temperature (°C)
BMIM-PF₆	10⁻¹⁰ Pa	10⁻¹⁵	7.5×10⁻¹³	20
Octaethylene glycol	9.2×10⁻⁸ Pa	9.2×10⁻¹³	6.9×10⁻¹⁰	89.85
Glycerol	0.4 Pa	0.000004	0.003	50
Mercury	1 Pa	0.00001	0.0075	41.85
Tungsten	1 Pa	0.00001	0.0075	3203
Xenon difluoride	600 Pa	0.006	4.50	25
Water (H₂O)	2.3 kPa	0.023	17.5	20
Propanol	2.4 kPa	0.024	18.0	20
Methyl isobutyl ketone	2.66 kPa	0.0266	19.95	25
Iron pentacarbonyl	2.80 kPa	0.028	21	20
Ethanol	5.83 kPa	0.0583	43.7	20
Freon-113	37.9 kPa	0.379	284	20
Acetaldehyde	98.7 kPa	0.987	740	20
Butane	220 kPa	2.2	1650	20
Formaldehyde	435.7 kPa	4.357	3268	20
Propane	997.8 kPa	9.978	7584	26.85
Carbonyl sulfide	1.255 MPa	12.55	9412	25
Nitrous oxide	5.660 MPa	56.60	42453	25
Carbon dioxide	5.7 MPa	57	42753	20

Estimating vapor pressure from molecular structure

Scientists have created ways to guess the vapor pressure of organic molecules just by looking at their structure. Some of these methods include the SIMPOL.1 method, the method by Moller and others, and EVAPORATION, which stands for Estimation of VApour Pressure of ORganics, taking into account things like temperature and molecule structure.

Meaning in meteorology

In meteorology, vapor pressure refers to the amount of water vapor in the air, even if it is not perfectly balanced. This is different from how the term is used in other sciences. The saturation vapor pressure is the pressure of water vapor when it is balanced above a flat surface of water or ice, and it depends only on the temperature. Relative humidity is measured compared to this saturation vapor pressure.

The term equilibrium vapor pressure can apply to tiny water droplets, possibly with impurities, such as in a cloud, and may differ from saturation vapor pressure based on the size of the droplets and other particles in the air. However, these terms are sometimes used in different ways by different authors. The idea of saturation vapor pressure comes from an old theory that air can only hold a certain amount of water vapor before it becomes "saturated." But according to Dalton's law, the pressure of water vapor depends only on the temperature of the liquid water, not on the air around it.