In chemistry, volatility is a material quality that describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapor, 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 a group of substances evaporate (or sublime 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 sublime (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.
Relations between vapor pressure, temperature, and boiling point
The vapor pressure of a substance is the pressure at which its gas phase is in equilibrium with its condensed phases (liquid or solid). It is a measure of the tendency of molecules and atoms to escape from a liquid or a solid. A liquid's atmospheric pressure boiling point corresponds to the temperature at which its vapor pressure is equal to the surrounding atmospheric pressure and it is often called the normal boiling point.
The higher the vapor pressure of a liquid at a given temperature, the higher the volatility and the lower the normal boiling point of the liquid. The vapor pressure chart (right hand side) displays the vapor pressures dependency for a variety of liquids as a function of temperature.
For example, at any given temperature, chloromethane (methyl chloride) has the highest vapor pressure of any of the liquids in the chart. It also has the lowest normal boiling point (−24.2 °C), which is where the vapor pressure curve (the blue line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapor pressure.
- Clausius–Clapeyron relation
- Fractional distillation
- Partial pressure
- Raoult's law
- Relative volatility
- Vapor–liquid equilibrium
- Volatile organic compound
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