Volatility (physics)
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In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, volatility is a material quality which describes how readily a substance vaporizes. At a given
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
and
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
, a substance with high volatility is more likely to exist as a
vapour In physics, a vapor (American English) or vapour (Commonwealth English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Herring, ''General ...
, while a substance with low volatility is more likely to be a
liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...
or
solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...
. Volatility can also describe the tendency of a vapor to
condense Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...
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 Rubbing alcohol, known as surgical spirit in the British Pharmacopoeia, refers to a group of denatured alcohol solutions commonly used as topical disinfectant. In addition to its medical applications, rubbing alcohol is employed in various indust ...
(
isopropyl alcohol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable, organic compound with a pungent alcoholic odor. Isopropyl alcohol, an organic polar molecule, is miscible in water, ethanol, an ...
) will quickly evaporate, while a substance with low volatility such as
vegetable oil Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of edible plants. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed ...
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 Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and Sublimation (phase transition), sublimes directly from the solid state to the gas ...
(solid carbon dioxide) or
iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...
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 The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envi ...
(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 Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...
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 The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envi ...
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 Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...
, 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 An intermolecular force (IMF; also secondary force) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction or repulsion which act between atoms and other types of neighbouring particles (e.g. ...
, such as most solids, are typically not very volatile.
Ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
and
dimethyl ether Dimethyl ether (DME; also known as methoxymethane) is the organic compound with the formula CH3OCH3, (sometimes ambiguously simplified to C2H6O as it is an isomer of ethanol). The simplest ether, it is a colorless gas that is a useful precursor ...
, 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 In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
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 The molecular mass () is the mass of a given molecule, often expressed in units of daltons (Da). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quan ...
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 In chemistry, an ester is a chemical compound, compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds c ...
, alkanes, etc.). For instance, linear
alkanes In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whi ...
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 Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
. The process of petroleum refinement utilizes a technique known as
fractional distillation Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. It uses distillation ...
, which allows several chemicals of varying volatility to be separated in a single step.
Crude oil Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring u ...
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 Butane () is an alkane with the formula C4H10. Butane exists as two isomers, ''n''-butane with connectivity and iso-butane with the formula . Both isomers are highly flammable, colorless, easily liquefied gases that quickly vaporize at ro ...
and
kerosene Kerosene, or paraffin, is a combustibility, combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in Aviation fuel, aviation as well as households. Its name derives from the Greek (''kērós'') meaning " ...
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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
has long been used to produce concentrated
alcoholic beverages Drinks containing alcohol are typically divided into three classes—beers, wines, and spirits—with alcohol content typically between 3% and 50%. Drinks with less than 0.5% are sometimes considered non-alcoholic. Many societies have a di ...
(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
perfume Perfume (, ) is a mixture of fragrance, fragrant essential oils or aroma compounds (fragrances), Fixative (perfumery), fixatives and solvents, usually in liquid form, used to give the human body, animals, food, objects, and living-spaces an agre ...
s. Humans detect
odor An odor (American English) or odour ( Commonwealth English; see spelling differences) is a smell or a scent caused by one or more volatilized chemical compounds generally found in low concentrations that humans and many animals can perceive ...
s 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 An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
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

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References


External links


Volatility from ilpi.com
{{States of matter Physical chemistry Chemical properties Thermodynamic properties Engineering thermodynamics Phase transitions Gases