Distillation is a physical process used to separate the components of a liquid mixture or to purify a liquid by removing contaminants. The technique involves converting a liquid into a vapor, then cooling the vapor to condense it back into a purified liquid form called the distillate. Today, distillation remains a fundamental operation across science and industry, used in everything from chemical manufacturing to water purification. The continuous repetition of evaporation and condensation allows for the separation of substances based entirely on their physical properties.
The Underlying Principle of Distillation
The power of distillation to separate substances is rooted in volatility, which is the tendency of a substance to vaporize. Every liquid exerts a vapor pressure, and as a mixture is heated, the component with the higher vapor pressure will transition into a gas more readily. This corresponds to the substance having a lower boiling point, which is the temperature at which its vapor pressure equals the surrounding atmospheric pressure. By carefully controlling the heat applied to the mixture, the substance with the lowest boiling temperature can be preferentially converted into a gas.
When the temperature of the mixture reaches the boiling point of the most volatile component, that substance begins to vaporize and ascend away from the liquid mixture. The resulting vapor phase is therefore richer in the more volatile component than the original liquid mixture. This vapor is then collected and passed through a condenser, a cooled pathway that causes the gas molecules to lose energy and revert to a liquid state. This newly formed, more concentrated liquid is collected separately, achieving the desired separation. Simple distillation is generally effective only when the boiling points of the components differ by a significant margin, often a difference of 25°C or more.
Separating Mixtures of Different Liquids
The most complex application of distillation involves separating two or more liquids that are fully mixed together. For mixtures where the boiling points are relatively close, simple distillation is ineffective because both components vaporize simultaneously, resulting in an impure distillate. To solve this problem, fractional distillation is employed, which involves repeated cycles of vaporization and condensation.
Fractional distillation utilizes a vertical column, often packed with materials like glass beads or metal rings, placed between the boiling flask and the condenser. As the mixed vapor travels up the column, it repeatedly condenses on the packing material and then re-vaporizes as it is reheated by the rising vapor from below. With each cycle of condensation and revaporization, the vapor becomes progressively richer in the more volatile, lower-boiling component. By the time the vapor reaches the top of the fractionating column, it is almost entirely composed of the desired, purer substance.
This method is routinely used in the petrochemical industry to refine crude oil, which is a complex mixture of hydrocarbon liquids. Crude oil is heated in a fractionating tower, separating it into distinct products at different temperature levels:
- Gasoline
- Kerosene
- Diesel fuel
- Lubricating oils
The purification of alcoholic spirits, such as whiskey or brandy, relies on fractional distillation to separate the lower-boiling ethanol (boiling point 78.4°C) from the higher-boiling water (boiling point 100°C). This process concentrates the alcohol content to create a potable spirit.
Removing Solids and Non-Volatile Components from Liquids
Distillation is a highly effective method for purifying a liquid by removing any component that is non-volatile, meaning it does not readily turn into a gas when heated. In this simple process, the liquid component is the only one that evaporates, leaving all the impurities behind in the original container. The non-volatile substances include dissolved solids, such as salts and minerals, as well as heavy oils, greases, and large organic molecules.
A common example is the use of distillation for water purification and desalination. Seawater, which contains a high concentration of dissolved salts, is heated until the water boils and turns into steam. The salt, being non-volatile, remains behind as a solid residue while the pure water vapor moves on to the condenser. Once condensed, the resulting liquid is highly purified, distilled water free of salts, heavy metals, and most other dissolved contaminants.