Vapor Pressure

By Tabitha Mishra
Last updated: April 2, 2022

What Does Vapor Pressure Mean?

Vapor pressure is pessure exerted by a substance in gas form. It results from the evaporation of a liquid or solid in a closed container. An increase in the temperature of the substance will result in an increase in the vapor pressure.

The vapor pressure of a liquid can be measured by injecting a small quantity of the liquid into a closed flask connected to a manometer. The unit of vapor pressure is the standard atmosphere (atm), kPa, or bar, with the following conversions:

1 atm = 101.325 kPa = 1.01325 bar

Safeopedia Explains Vapor Pressure

Rate of Evaporation of a Substance

The rate of evaporation indicates how quickly a liquid or solid turns into vapor at room temperature.

The evaporation rate is given in comparison with chemicals that evaporate quickly, like butyl acetate. For instance, if for butyl acetate = 1, the rate of evaporation of a substance is given as 0.5, this means that under the same conditions, 0.5 grams of the substance will evaporate at the same rate as 1 gram of butyl acetate.

Occupational Safety and Chemical Volatility

Vapor pressure is an indication of the volatility of a chemical substance. It is used by regulatory authorities to determine the risk of occupational exposure and release to the environment by predicting its concentration in the air. It informs whether a substance is available for inhalation as a vapor.

Chemicals with a higher vapor pressure are more likely to enter the air and are to be handled in ways that minimize their emissions. Volatile substances with high vapor pressure and high evaporation rates also have low flash points, making them highly flammable.

Flashpoint and Volatility

Flammable liquids are classified according to their individual flashpoints, since this directly relates to their volatility and ability to generate vapor. Vapor, in turn, is the primary factor in determining a fire hazard as it is the vapor that burns rather than the substance itself.

The flashpoint is the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. The liquid in the test vessel is heated and the temperature is noted at which a flash is obtained when a small flame is introduced into the vapor zone above the surface of the substance.


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