A volatile substance is one which evaporates readily. Well-known examples are gasoline and dry ice. Although a qualitative concept, it has found considerable application in the development of chemistry as a science.

In the early years of chemistry, as scientists sorted out basic concepts, the term volatile was applied to materials which would yield gases upon various chemical treatments. A common substance which was widely studied was sal volatile, also known as volatile alkali. This substance is known today as ammonium carbonate. Ammonium carbonate is a solid, but chemical treatment can result in its breakdown, releasing ammonia and carbon dioxide, both gases under normal conditions. Such a disappearance of a solid into the gas phase was an intriguing phenomenon. Two prominent early chemists, Robert Boyle and Joseph Black were among those who studied the reactions of volatile alkali. Boyle also wrote of volatile nitre (saltpeter) which is either potassium or sodium nitrate. Under certain treatments, saltpeter yields nitrogen gas. Antoine-Laurent Lavosier experimented with volatile sulphurous acid which readily yields the gas sulfur dioxide.

In present usage, the volatility of a substance refers to the relative ease with which it can be vaporized. Quantitative measures of volatility include vapor pressure and boiling point. Substances with high vapor pressures are highly volatile. Since the boiling point of a substance is the temperature at which its vapor pressure is equal to the atmospheric pressure, a substance whose boiling point is lower than another's is said to be more volatile. For instance, bromine and mercury both exist as liquids at room temperature, but bromine boils at 138°F (59°C) while mercury's boiling point is 693°F (367°C). Bromine is considerably more volatile than mercury. The boiling points of some common materials regarded as having high volatility are: chloroform, 142°F (61°C); ether, 95°F (35°C); and gasoline, 158-194°F (70-90°C).