In chemistry, a volatile substance is one which readily vaporizes. The term “volatile” comes from the Latin word volare, meaning to fly, and it describes the tendency of a substance to vaporize in air. Volatility is a measure of how easily a material transitions into the gas phase via evaporation (liquids) or sublimation (solids). Volatility is not measured directly, but a volatile substance has a high vapor pressure and a low boiling point. Whether or not a substance is volatile has important implications in chemistry and also in perfumes, cooking, and industrial process.
How Volatility Works
The volatility of a substance is determined by its vapor pressure, which is the pressure exerted by the vapor of a substance in equilibrium with its liquid or solid phase. Substances with high vapor pressures have a greater tendency to evaporate, while those with low vapor pressures tend to remain in the liquid or solid phase. The relationship between vapor pressure and temperature is described by the Clausius-Clapeyron equation, which states that the vapor pressure of a substance increases with increasing temperature.
Meanwhile, decreasing pressure lowers the boiling point of a substance. Volatile substance tend to have a low boiling point.
Examples of Volatile Substances
Volatile substances include elements as well as compounds. Many volatile chemicals are organic, but some are inorganic.
- Mercury (Hg) and bromine (Br) are volatile elements that are liquids at ordinary temperature and pressure. Both elements have high vapor pressures and readily release particles into the air above the liquid surface. In contrast, iron and silicon are solid at room temperature and are not volatile.
- Dry ice is the solid form of carbon dioxide (CO2). It is volatile at room temperature and normal pressure because it readily sublimates into carbon dioxide gas.
- Osmium tetroxide (OsO4) is another volatile inorganic compound. Like dry ice, it undergoes sublimation from a solid directly into vapor.
- Alcohol, xylene, benzene, and toluene are examples of volatile organic compounds commonly used as solvents.
- Methane (CH4) and molecular hydrogen (H2) are highly volatile. So are gasoline, propane, kerosene, and most other fuels.
- Ammonia, hydrogen peroxide, and formaldehyde are volatile.
- Perfume contains volatile compounds that vaporize in air so you smell their scent.
Factor That Affect Whether a Substance is Volatile
Two significant factors that determine whether or not a substance is volatile are molecular weight and intermolecular forces.
Volatility generally decreases with increasing molecular weight. Basically, the more atoms a molecule has, the more likely it participates in intermolecular bonding. For example, linear alkane become less volatile as the size of the carbon chain increases.
Intermolecular forces also matter. Water (H2O) readily forms hydrogen bonds with other water molecules. So, even though it is a small molecule, its polarity and hydrogen bonding give it a low vapor pressure compared to nonpolar molecules of the same size. As another example, ethanol and dimethyl ether both have the same chemical formula (C2H6O), but different structures. Ethanol molecules experience hydrogen bonding while dimethyl ether molecules do not. Both substance are volatile, but dimethyl ether is more volatile than ethanol.
Applications of Volatility
Volatile substances are important in everyday life, chemistry, and industry. Volatile organic compounds in foods and beverages give them characteristic odors. For example, you smell brewing coffee or frying bacon. Sometimes the odors protect you from eating spoiled food, like rotten eggs or “off” milk. Perfumes use volatile compounds to produce pleasing scents. Chemists apply volatility when they select solvents that readily vaporize for chemical synthesis and distillation. In industry, fuel volatility is carefully adjusted for optimal evaporation and ignition.
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