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A volatile substance is any solid or liquid that has a high vapor pressure at room temperature, so it evaporates quickly . Volatile substances in general should be stored in airtight containers to prevent them from escaping.
There are many volatile substances to which we are constantly exposed. For example, the organic solvents in which nail polishes are prepared are always volatile substances. In fact, they are chosen as solvents precisely because of their rapid evaporation, which ensures fast drying for the enamel.
Some fuels, such as high octane gasoline, are considerably volatile; There are also examples of some solid substances that have a very high vapor pressure that allows them to spontaneously sublimate.
Characteristics of volatile substances
- They have a high vapor pressure.
- They have low boiling points.
- In general, they have low intermolecular forces of interaction, so they do not bind strongly to each other.
- They are not usually polar molecules nor are they usually capable of forming hydrogen bonds with each other.
- In general, they have a low molecular weight.
- Most are liquid at room temperature, although some are solid.
vapor pressure and evaporation rate
The concept of volatility in chemistry has to do with vapor pressure. Vapor pressure is defined as the pressure of a substance in the gas phase that is in dynamic equilibrium with the liquid or solid phase .
Vapor pressure is a measure of the rate at which a substance evaporates, as it indicates the pressure required to increase the rate of condensation or deposition until it equals the rate of evaporation. If a very high pressure is needed (that is, if the vapor pressure is high), it means that a high condensation rate is needed because the evaporation rate is high.
Examples of Volatile Substances
Ethyl ether – C 2 H 5 OC 2 H 5
Ethers in general are very volatile compounds. The simplest of them, dimethyl ether, at room temperature is not even a liquid, but a gas. But ethyl ether is a liquid, and it has a pressure of about 0.7 atmospheres (almost atmospheric pressure). If it were only about 0.3 atmospheres larger, then it would also be a gas at room temperature.
Solid iodine – I 2
Solid iodine is accepted by the scientific community as a volatile solid. In fact, this halogen sublimes rather than melts and in any sealed container containing iodine you can see the gaseous iodine as a small purple cloud. However, the vapor pressure of iodine is only 0.027 kPa (0.000266 atm) at 20ºC. This pressure is only a tiny fraction of the vapor pressure of most liquids. Despite this, iodine is considered a volatile solid since, small as it may seem, that vapor pressure is in fact much higher than that of the vast majority of solids.
petroleum ether
Despite its name, petroleum ether is not actually an ether from a chemical point of view. It is a very light and highly volatile (hence its name, which means “upper air”) fraction of petroleum distillation that contains multiple short-chain hydrocarbons. The boiling point is always in the range of 30 to 60 ºC, so it is almost a gas at room temperature.
Liquid bromine – Br 2
Bromine (Br 2 ) is a highly volatile liquid halogen. Its vapor pressure is 0.30 atm which makes it evaporate quickly unless it is kept in a properly sealed container.
Absolute methanol – CH 3 OH
Used in some cases as jet fuel and in others as race car fuel, the simplest alcohol has a very high vapor pressure, making it a fairly volatile liquid. At a temperature of 37.8ºC its vapor pressure is 0.32 atm.
The gas
It is a complex mixture of alkanes that mainly include different octane isomers (alkane with 8 carbon atoms). The vapor pressure of gasoline is in the order of 0.60 atm, which is very high.
Electronic circuit cleaner spray
They are mixtures of volatile organic compounds that include petroleum distillates (with alkanes such as heptane, propane, and cyclohexane) and low molecular weight alcohols such as ethanol, isopropanol, and mineral spirits. The vapor pressure of the mixture is around 1 mmHg, so it evaporates very quickly after being sprayed on the circuit to be cleaned.
Tungsten hexafluoride – WCl 6
This compound has a melting point of just 2.3ºC and a boiling point of only 17.1ºC, so technically it is neither a solid nor a liquid at a standard temperature of 25ºC, but it is a substance very volatile. In fact, it is one of the heaviest gases known. However, both the liquid and the solid both have very high vapor pressures at 20ºC, exceeding atmospheric pressure (that is why it is gaseous at that temperature).
Hexacarbonyl tungsten – W(CO) 6
This is the heavy cousin of the hexafluoride just shown. At 67 ºC this compound has a vapor pressure almost 5 times greater than that of solid iodine at room temperature. This compound also sublimates instead of melting under standard pressure.
volatility and temperature
The reason hair dryers blow hot air is because the heat helps water evaporate more quickly. This means that the higher the temperature, the more volatile the water becomes. This happens to most substances and the reason is that the higher the temperature, the higher its vapor pressure. In fact, if the temperature is increased a lot, the vapor pressure can become equal to the atmospheric pressure, in which case the boiling point (in the case of liquids) or the sublimation point (in the case of liquids) is reached. the solids).
References
Gaspar, DJ, Phillips, SD, Polikarpov, E., Albrecht, KO, Jones, SB, George, A., . . . Bays, J.T. (2019). Measuring and predicting the vapor pressure of gasoline containing oxygenates. Fuel , 243 , 630–644. https://doi.org/10.1016/j.fuel.2019.01.137
List of volatile organic compounds. (2021). Recovered from https://condorchem.com/es/listado-compuestos-organicos-volatiles/
Sublimation of iodine: Rise and fall of a misconception | Chem13 News. (2019, September 10). Retrieved from https://uwaterloo.ca/chem13-news-magazine/october-2015/feature/sublimation-iodine-rise-and-fall-misconception.
Vernon, A.A. (1937). The Vapor Pressure and Dissociation of Tungsten Hexachloride in the Gas Phase1. Journal of the American Chemical Society , 59 (10), 1832–1833. https://doi.org/10.1021/ja01289a013
