Distillation is a method for separating the components of a liquid mixture based on differences in their physical properties. This thermal separation technique is a physical process, meaning it causes a change in state but not a chemical alteration of the substances involved. Distillation remains a fundamental procedure in modern laboratories and industrial settings globally for achieving high purity.
Defining the Process
Distillation works by taking advantage of the difference in volatility between the substances in a liquid mixture. Volatility describes how readily a substance transitions from a liquid phase into a gaseous phase, or vapor. The purpose of the process is either to isolate a specific, pure liquid component, known as the distillate, or to remove non-volatile contaminants from the original substance.
Separation relies on the concept of a boiling point, the specific temperature at which a liquid’s internal vapor pressure equals the external atmospheric pressure. When a mixture is heated, the component with the lower boiling point will convert into a vapor first, preferentially leaving the other, less volatile substances behind.
This method is highly effective when separating a liquid from dissolved, non-volatile solids, such as purifying water from salt. It is also used for liquid mixtures, though the components must have reasonably different boiling points for a simple distillation to be effective. If the boiling points of the liquids are too close, more complex variations of the technique are necessary to achieve sufficient separation.
The Science of Separation
The physical act of distillation is a continuous cycle involving three distinct stages: vaporization, condensation, and collection. This process begins when the liquid mixture is placed into a vessel, often a distillation flask or still, which is then subjected to a regulated heat source.
The first stage, vaporization, occurs as the heated substance converts into a gas. The molecules of the liquid with the lowest boiling point gain enough thermal energy to escape the liquid phase and rise as vapor. This vapor, now richer in the desired volatile component, is directed away from the liquid mixture.
This gaseous substance then flows into the second stage, which is the condenser. A condenser is typically a glass tube surrounded by a jacket of continuously flowing cold water, which provides a cold surface area. As the hot vapor molecules make contact with this chilled surface, they lose thermal energy rapidly, causing them to revert, or condense, back into a liquid state.
The final stage is the collection of the purified liquid, now called the distillate. The liquid drips from the condenser tube and is collected in a separate receiving vessel, isolated from the original mixture. For separating liquids with very similar boiling points, a fractionating column is often inserted, which forces the vapor to undergo multiple cycles of vaporization and condensation to increase the purity of the final product.
Everyday Applications
One familiar application is the production of alcoholic spirits, such as whiskey, vodka, and rum. Fermented liquids, like wine or beer mash, contain ethyl alcohol, which has a boiling point of approximately 78.37°C, significantly lower than water’s 100°C. Heating the fermented mixture allows the alcohol to vaporize before the water, effectively concentrating the ethanol content in the final collected distillate. This process is repeated multiple times to achieve the higher alcohol concentrations found in spirits.
The process is also employed extensively in water purification, particularly for desalination. Heating impure water causes the water to vaporize while leaving behind non-volatile contaminants like dissolved salts, minerals, and heavy metals. The resulting condensed liquid is highly pure distilled water, which is used for drinking in areas with limited fresh water access or for sensitive industrial applications like laboratory work and medical equipment.
Perhaps the largest industrial application is the refining of crude oil, which is separated into various petroleum products through a process called fractional distillation. Crude oil is a complex blend of hydrocarbons, each possessing a different boiling range based on its molecular size. The oil is heated and the resulting vapors rise through a tall column, condensing at different temperature levels to be collected as distinct products. This physical separation yields fuels like gasoline, diesel, and jet fuel, which are separated based on their specific boiling properties.