Volatility is the ease with which a substance is converted to the gaseous, or vapor, state. The term is usually used to describe the speed with which a liquid evaporates, but it can also apply to the process of a solid changing to a gas, known as sublimation. Liquids that boil at low temperatures, such as gasoline, are volatile liquids, while liquids that boil at higher temperatures, such as water, are less volatile or nonvolatile. Extremely volatile substances have such low boiling points that they exist as gases at room temperature, such as oxygen gas.
Several factors affect the volatility of a liquid. In general, larger molecules are less volatile than smaller ones among molecules that share similar composition and construction. For example, ethyl alcohol is more volatile and evaporates more readily than a larger alcohol such as decanol. Almost every chemical is more volatile at higher temperatures than at lower ones. Water, which evaporates faster on a hot day than on a cold one, is a good example of this. Volatility is also affected by atmospheric pressure. Liquids are more volatile at higher altitudes, where atmospheric pressure is less. Some substances that are liquids at sea level will evaporate very quickly on top of a high mountain, assuming no change in other factors such as temperature.
Some compounds, such as water, are extremely non-volatile; often this is because of strong chemical bonds between the molecules—the most common of which are hydrogen bonds—that resist the tendencies of individual molecules to enter the gaseous state.