In chemistry, volatility is a material quality which describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapour, while a substance with low volatility is more likely to be a liquid or solid. Volatility can also describe the tendency of a vapor to condense into a liquid or solid; less volatile substances will more readily condense from a vapor than highly volatile ones. Differences in volatility can be observed by comparing how fast substances within a group evaporate (or sublimate in the case of solids) when exposed to the atmosphere. A highly volatile substance such as rubbing alcohol (isopropyl alcohol) will quickly evaporate, while a substance with low volatility such as vegetable oil will remain condensed. In general, solids are much less volatile than liquids, but there are some exceptions. Solids that sublimate (change directly from solid to vapor) such as dry ice (solid carbon dioxide) or iodine can vaporize at a similar rate as some liquids under standard conditions.
Description
Volatility itself has no defined numerical value, but it is often described using vapor pressures or boiling points (for liquids). High vapor pressures indicate a high volatility, while high boiling points indicate low volatility. Vapor pressures and boiling points are often presented in tables and charts that can be used to compare chemicals of interest. Volatility data is typically found through experimentation over a range of temperatures and pressures.
Vapor pressure
Vapor pressure is a measurement of how readily a condensed phase forms a vapor at a given temperature. A substance enclosed in a sealed vessel initially at vacuum (no air inside) will quickly fill any empty space with vapor. After the system reaches equilibrium and the rate of evaporation matches the rate of condensation, the vapor pressure can be measured. Increasing the temperature increases the amount of vapor that is formed and thus the vapor pressure. In a mixture, each substance contributes to the overall vapor pressure of the mixture, with more volatile compounds making a larger contribution.
Boiling point
Boiling point is the temperature at which the vapor pressure of a liquid is equal to the surrounding pressure, causing the liquid to rapidly evaporate, or boil. It is closely related to vapor pressure, but is dependent on pressure. The normal boiling point is the boiling point at atmospheric pressure, but it can also be reported at higher and lower pressures.
Contributing factors
Intermolecular forces
An important factor influencing a substance's volatility is the strength of the interactions between its molecules. Attractive forces between molecules are what holds materials together, and materials with stronger intermolecular forces, such as most solids, are typically not very volatile. Ethanol and dimethyl ether, two chemicals with the same formula (C2H6O), have different volatilities due to the different interactions that occur between their molecules in the liquid phase: ethanol molecules are capable of hydrogen bonding while dimethyl ether molecules are not. The result in an overall stronger attractive force between the ethanol molecules, making it the less volatile substance of the two.
Molecular weight
In general, volatility tends to decrease with increasing molecular mass because larger molecules can participate in more intermolecular bonding, although other factors such as structure and polarity play a significant role. The effect of molecular mass can be partially isolated by comparing chemicals of similar structure (i.e. esters, alkanes, etc.). For instance, linear alkanes exhibit decreasing volatility as the number of carbons in the chain increases.
Applications
Distillation
Knowledge of volatility is often useful in the separation of components from a mixture. When a mixture of condensed substances contains multiple substances with different levels of volatility, its temperature and pressure can be manipulated such that the more volatile components change to a vapor while the less volatile substances remain in the liquid or solid phase. The newly formed vapor can then be discarded or condensed into a separate container. When the vapors are collected, this process is known as distillation.
The process of petroleum refinement utilizes a technique known as fractional distillation, which allows several chemicals of varying volatility to be separated in a single step. Crude oil entering a refinery is composed of many useful chemicals that need to be separated. The crude oil flows into a distillation tower and is heated up, which allows the more volatile components such as butane and kerosene to vaporize. These vapors move up the tower and eventually come in contact with cold surfaces, which causes them to condense and be collected. The most volatile chemical condense at the top of the column while the least volatile chemicals to vaporize condense in the lowest portion.
The difference in volatility between water and ethanol has long been used to produce concentrated alcoholic beverages (many of these are referred to as "liquors"). In order to increase the concentration of ethanol in the product, beverage makers would heat the initial alcohol mixture to a temperature where most of the ethanol vaporizes while most of the water remains liquid. The ethanol vapor is then collected and condensed in a separate container, resulting in a much more concentrated product.
Perfume
Volatility is an important consideration when crafting perfumes. Humans detect odors when aromatic vapors come in contact with receptors in the nose. Ingredients that vaporize quickly after being applied will produce fragrant vapors for a short time before the oils evaporate. Slow-evaporating ingredients can stay on the skin for weeks or even months, but may not produce enough vapors to produce a strong aroma. To prevent these problems, perfume designers carefully consider the volatility of essential oils and other ingredients in their perfumes. Appropriate evaporation rates are achieved by modifying the amount of highly volatile and non-volatile ingredients used.
See also
- Clausius–Clapeyron relation – Relation between vapour pressure and temperature
- Distillation – Method of separating mixtures
- Fractional distillation – Separation of a mixture into its component parts
- Partial pressure – Pressure of a component gas in a mixture
- Raoult's law – Law of thermodynamics for vapour pressure of a mixture
- Relative volatility – Comparative volatility of components in a solution
- Vapor–liquid equilibrium – Concentration of a vapor in contact with its liquid
- Volatile organic compound – Organic chemicals having a high vapor pressure at room temperature
References
- Felder, Richard (2015). Elementary Principles of Chemical Processes. John Wiley & Sons. pp. 279–281. ISBN 978-1-119-17764-7.
- Koretsky, Milo D. (2013). Engineering and Chemical Thermodynamics. John Wiley & Sons. pp. 639–641.
- Zumdahl, Steven S. (2007). Chemistry. Houghton Mifflin. pp. 460-466. ISBN 978-0-618-52844-8.
- Atkins, Peter (2013). Chemical Principles. New York: W.H. Freeman and Company. pp. 368–369. ISBN 978-1-319-07903-1.
- "Hydrocarbon boiling points". Archived from the original on 7 February 2023. Retrieved 28 April 2021.
- Armarego, Wilfred L. F. (2009). Purification of Laboratory Chemicals. Elsevier. pp. 9-12. ISBN 978-1-85617-567-8.
- Kvaalen, Eric. "Alcohol Distillation: Basic Principles, Equipment, Performance Relationships, and Safety". .
- Sell, Charles (2006). The Chemistry of Fragrances. UK: The Royal Society of Chemistry. pp. 200-202. ISBN 978-0-85404-824-3.
External links
- Volatility from ilpi.com
In chemistry volatility is a material quality which describes how readily a substance vaporizes At a given temperature and pressure a substance with high volatility is more likely to exist as a vapour while a substance with low volatility is more likely to be a liquid or solid Volatility can also describe the tendency of a vapor to condense into a liquid or solid less volatile substances will more readily condense from a vapor than highly volatile ones Differences in volatility can be observed by comparing how fast substances within a group evaporate or sublimate in the case of solids when exposed to the atmosphere A highly volatile substance such as rubbing alcohol isopropyl alcohol will quickly evaporate while a substance with low volatility such as vegetable oil will remain condensed In general solids are much less volatile than liquids but there are some exceptions Solids that sublimate change directly from solid to vapor such as dry ice solid carbon dioxide or iodine can vaporize at a similar rate as some liquids under standard conditions Bromine liquid readily transitions to vapor at room temperature indicating high volatility DescriptionVolatility itself has no defined numerical value but it is often described using vapor pressures or boiling points for liquids High vapor pressures indicate a high volatility while high boiling points indicate low volatility Vapor pressures and boiling points are often presented in tables and charts that can be used to compare chemicals of interest Volatility data is typically found through experimentation over a range of temperatures and pressures Vapor pressure A log lin vapor pressure chart for various liquids Vapor pressure is a measurement of how readily a condensed phase forms a vapor at a given temperature A substance enclosed in a sealed vessel initially at vacuum no air inside will quickly fill any empty space with vapor After the system reaches equilibrium and the rate of evaporation matches the rate of condensation the vapor pressure can be measured Increasing the temperature increases the amount of vapor that is formed and thus the vapor pressure In a mixture each substance contributes to the overall vapor pressure of the mixture with more volatile compounds making a larger contribution Boiling point Boiling point is the temperature at which the vapor pressure of a liquid is equal to the surrounding pressure causing the liquid to rapidly evaporate or boil It is closely related to vapor pressure but is dependent on pressure The normal boiling point is the boiling point at atmospheric pressure but it can also be reported at higher and lower pressures Contributing factorsIntermolecular forces Normal boiling point red and melting point blue of linear alkanes vs number of carbon atoms An important factor influencing a substance s volatility is the strength of the interactions between its molecules Attractive forces between molecules are what holds materials together and materials with stronger intermolecular forces such as most solids are typically not very volatile Ethanol and dimethyl ether two chemicals with the same formula C2H6O have different volatilities due to the different interactions that occur between their molecules in the liquid phase ethanol molecules are capable of hydrogen bonding while dimethyl ether molecules are not The result in an overall stronger attractive force between the ethanol molecules making it the less volatile substance of the two Molecular weight In general volatility tends to decrease with increasing molecular mass because larger molecules can participate in more intermolecular bonding although other factors such as structure and polarity play a significant role The effect of molecular mass can be partially isolated by comparing chemicals of similar structure i e esters alkanes etc For instance linear alkanes exhibit decreasing volatility as the number of carbons in the chain increases ApplicationsDistillation A crude oil distillation tower Knowledge of volatility is often useful in the separation of components from a mixture When a mixture of condensed substances contains multiple substances with different levels of volatility its temperature and pressure can be manipulated such that the more volatile components change to a vapor while the less volatile substances remain in the liquid or solid phase The newly formed vapor can then be discarded or condensed into a separate container When the vapors are collected this process is known as distillation The process of petroleum refinement utilizes a technique known as fractional distillation which allows several chemicals of varying volatility to be separated in a single step Crude oil entering a refinery is composed of many useful chemicals that need to be separated The crude oil flows into a distillation tower and is heated up which allows the more volatile components such as butane and kerosene to vaporize These vapors move up the tower and eventually come in contact with cold surfaces which causes them to condense and be collected The most volatile chemical condense at the top of the column while the least volatile chemicals to vaporize condense in the lowest portion The difference in volatility between water and ethanol has long been used to produce concentrated alcoholic beverages many of these are referred to as liquors In order to increase the concentration of ethanol in the product beverage makers would heat the initial alcohol mixture to a temperature where most of the ethanol vaporizes while most of the water remains liquid The ethanol vapor is then collected and condensed in a separate container resulting in a much more concentrated product Perfume Volatility is an important consideration when crafting perfumes Humans detect odors when aromatic vapors come in contact with receptors in the nose Ingredients that vaporize quickly after being applied will produce fragrant vapors for a short time before the oils evaporate Slow evaporating ingredients can stay on the skin for weeks or even months but may not produce enough vapors to produce a strong aroma To prevent these problems perfume designers carefully consider the volatility of essential oils and other ingredients in their perfumes Appropriate evaporation rates are achieved by modifying the amount of highly volatile and non volatile ingredients used See alsoClausius Clapeyron relation Relation between vapour pressure and temperature Distillation Method of separating mixtures Fractional distillation Separation of a mixture into its component parts Partial pressure Pressure of a component gas in a mixture Raoult s law Law of thermodynamics for vapour pressure of a mixture Relative volatility Comparative volatility of components in a solution Vapor liquid equilibrium Concentration of a vapor in contact with its liquid Volatile organic compound Organic chemicals having a high vapor pressure at room temperatureReferencesFelder Richard 2015 Elementary Principles of Chemical Processes John Wiley amp Sons pp 279 281 ISBN 978 1 119 17764 7 Koretsky Milo D 2013 Engineering and Chemical Thermodynamics John Wiley amp Sons pp 639 641 Zumdahl Steven S 2007 Chemistry Houghton Mifflin pp 460 466 ISBN 978 0 618 52844 8 Atkins Peter 2013 Chemical Principles New York W H Freeman and Company pp 368 369 ISBN 978 1 319 07903 1 Hydrocarbon boiling points Archived from the original on 7 February 2023 Retrieved 28 April 2021 Armarego Wilfred L F 2009 Purification of Laboratory Chemicals Elsevier pp 9 12 ISBN 978 1 85617 567 8 Kvaalen Eric Alcohol Distillation Basic Principles Equipment Performance Relationships and Safety Sell Charles 2006 The Chemistry of Fragrances UK The Royal Society of Chemistry pp 200 202 ISBN 978 0 85404 824 3 External linksLook up volatile or volatility in Wiktionary the free dictionary Volatility from ilpi com
