Volatility (chemistry)

In chemistry and physics, volatility is a term used to characterize the tendency of a substance to vaporize. 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 substance with a lower vapor pressure.

In common usage, the term applies primarily to liquids. However, it may also be used to characterize the process of sublimation by which certain solid substances such as ammonium chloride (NH₄Cl) and dry ice, which is solid carbon dioxide (CO₂), 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 °C (68 to 77 °F) is very often referred to as being volatile.

(PD) Image: Milton Beychok
Example vapor pressure graphs of various liquids.

Vapor pressure, temperature and boiling point

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 molecules and atoms 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 and also confirms that liquids with higher vapor pressures have lower normal boiling points.

For example, at any given temperature, methyl chloride (CH₃Cl) 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 (atm) of absolute vapor pressure.

In terms of intermolecular forces, the boiling point represents the point at which the liquid molecules possess enough thermal energy to overcome the various intermolecular attractions binding the molecules into the liquid. Therefore the boiling point is also an indicator of the strength of those attractive forces. The higher the intermolecular attractive forces are, the more difficult it is for molecules to escape from the liquid and hence the lower is the vapor pressure of the liquid. The lower the vapor pressure of the liquid, the higher the temperature must be to initiate boiling. Thus, the higher the intermolecular attractive forces are, the higher is the normal boiling point.

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 distillation processes. 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 two components) of a liquid mixture, the easier it is to separate these two components by distillation. By convention, relative volatility is typically denoted as α.

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 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 carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions (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.