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In [[chemistry]] and [[physics]], '''volatility''' is a term used to characterize the tendency of a substance to vaporize.<ref>'''Note:''' To vaporize means to become a vapor.</ref> It is directly related to a substance' s [[vapor pressure]]. At a given [[temperature]], a substance with a higher vapor pressure will vaporize more readily than a [[Gas|vapor]] with a lower vapor pressure.<ref>[http://www.bae.uky.edu/~snokes/BAE549thermo/gasesvapor.htm Gases and Vapor] ([[University of Kentucky]] website)</ref><ref>{{cite book|author=James G. Speight|title=The Chemistry and Technology of Petroleum|edition=4th Edition|publisher=CRC Press|date=2006|isbn=0-8493-9067-2}}</ref><ref name=Kister>{{cite book|author=Kister, Henry Z.|title=[[Distillation Design]]|edition=1st Edition|publisher=McGraw-Hill|year=1992|isbn=0-07-034909-6}}</ref>


In common usage, the term applies primarily to [[liquid]]s. However, it may also be used to characterize the process of  [[Sublimation (chemistry)|sublimation]] by which certain [[solid]] substances such as [[ammonium chloride]] (NH<sub>4</sub>Cl) and [[dry ice]], which is solid [[carbon dioxide]] (CO<sub>2</sub>), change directly from their solid form to a vapor without becoming a liquid.
Any substance with a significant vapor pressure at temperatures  of about 20 to 25 °[[Celsius (unit)|C]]  (68 to 77 °[[Fahrenheit (unit)|F]]) is very often referred to as being ''volatile''.
{{Image|Vapor Pressure Chart2.png|right|350px|Example vapor pressure graphs of various liquids.}}
== Vapor Pressure, temperature and boiling point ==
{{Main|Vapor pressure|boiling point|Antoine equation}}
The vapor pressure of a substance is the pressure at which its gaseous (vapor) phase is in equilibrium with its liquid or solid phase. It is a measure of the tendency of [[molecule]]s and [[atom]]s to escape from a liquid or solid.  A liquid's [[boiling point]] at [[atmospheric pressure]] corresponds to the temperature at which its vapor pressure is equal to the surrounding atmospheric pressure and is very commonly  referred to as the ''normal boiling point''.
The higher is the vapor pressure of a liquid, the higher is the volatility and the lower is the normal boiling point of the liquid. The adjacent vapor pressure chart graphs the dependency of vapor pressure upon temperature for a variety of liquids<ref name=Perry>{{cite book|author=R.H. Perry and D.W. Green (Editors)|title=Perry's Chemical Engineers' Handbook | edition=7th Edition|publisher=McGraw-Hill|year=1997|id=ISBN 0-07-049842-5}}</ref> and also confirms that liquids with higher vapor pressures have lower normal boiling points.
For example, at any given temperature, methyl chloride (CH<sub>3</sub>Cl) has the highest vapor pressure of any of the liquids graphed in the chart. It also has the lowest normal boiling point (−26 °C), which is where its vapor pressure curve (the blue line) intersects the horizontal pressure line of one [[atmosphere (unit)|atmosphere]] (atm) of absolute vapor pressure.
== Relative volatility ==
{{main|Relative volatility}}
''[[Relative volatility]]'' is a measure of the difference between the vapor pressure of the more volatile components of a liquid mixture and the vapor pressure of the less volatile components of the mixture.  This measure is widely used in designing large industrial [[Continuous distillation|distillation]] processes.<ref name=Kister/><ref name=Perry/><ref name=Seader>{{cite book|author= Seader, J. D., and Henley, Ernest J.|title=Separation Process Principles|publisher=Wiley| location=New York|year=1998|id=ISBN 0-471-58626-9}}</ref> In effect, it indicates the ease or difficulty of using distillation to separate the more volatile components from the less volatile components in a mixture. In other words, the higher is the relative volatility of a liquid mixture, the easier it is to separate the mixture components by distillation. By convention, relative volatility is typically denoted as <math>\alpha</math>.
== Volatile Organic Compound ==
{{main|Volatile organic compound}}
''[[Volatile organic compound]]'' (VOC) is a term that refers to organic chemical compounds having significant vapor pressures and which can have adverse effects on the [[Natural environment|environment]] and human health. VOCs are numerous, varied and include man-made (anthropogenic) as well as naturally occurring chemical compounds. The anthropogenic VOCs are regulated by various governmental environmental entities worldwide. 
There is no universally accepted definition of VOCs. Some regulatory entities define them in terms of their vapor pressure at ordinary temperatures, or their normal boiling points, or how many carbon atoms they contain per molecule, and others define them in terms of their [[photochemical reactivity]].
The [[U.S. Environmental Protection Agency]] currently defines them as any compound of [[carbon]], excluding [[carbon monoxide]], [[carbon dioxide]] , [[carbonic acid]], metallic [[carbide]]s or [[carbonate]]s, and [[ammonium carbonate]], which participates in [[atmospheric photochemical reaction]]s (i.e., the reactions that produce photochemical [[smog]]. However, any such carbon compounds that have been determined to have a low photochemical reactivity, and specifically listed in the regulation, are exempted from regulation.<ref>U.S. Code of Federal Regulations: 40 CFR 51.100(s) - Definition - Volatile organic compounds (VOC) </ref>
== References ==
{{reflist}}
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Bibliography:
*{{cite book|author=Lawrence K. Wang, Norman C. Pereira and Yung-Tse Hung (Editors)|title=Air Pollution Control Engineering|edition=1st Edition|publisher=Humana Press|year=2004|id=ISBN 1-58829-161-8}}

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