What Is Distillation?

Distillation is a chemical process that separates liquids into components. It’s been around for a long time – baked clay distillation tubes have been found at Taxila, Shaikhan Dheri and Charsadda in Pakistan and Rang Mahal in India dating from the early centuries of the Common Era.


Distillation uses heat to vaporize the component with the lowest boiling point, which then condenses into a collection container. This method can also remove lead, nitrates, iron, calcium and microorganisms from water.

Boiling Point

The point at which a liquid evaporates is called its boiling point. The higher a compound’s boiling point, the more volatile it is. A pure substance has a very high boiling point, but chemical mixtures have a range of boiling points. A distillation process separates a mixture into different substances by heating it and allowing the components with lower boiling points to evaporate first. The result is a liquid that contains the higher-boiling compounds and a vapor that contains the lower-boiling compounds.

The boiling point of a liquid depends on the ratio of its vapor pressure to atmospheric pressure. For example, water boils at 100 degrees C (212 degrees F), whereas salt does not. When a liquid is heated, its vapor pressure increases until it equals the external pressure of the atmosphere. Then the liquid will boil, forming a steam consisting of the non-volatile compounds. In a laboratory experiment, a distillation apparatus is set up with a heat source and a cooling system that includes a condenser. The cooling system must be carefully insulated to prevent air from entering the condenser and raising the temperature of the vapor.

When a mixture is distilled, the vapor passes through a series of condensers that are each set at a slightly lower temperature than the next one. As the vapor passes through each condenser, it cools and becomes a liquid again. Then the liquid can be separated into fractions based on its boiling point. For example, the fractions produced from a distillation of alcohol include ethyl acetate, which has a low boiling point, and hexyl acetate, which has s higher boiling point.

Vapor Pressure

Distillation exploits the differences in the volatilities of a solution’s components. The component with the lowest boiling point will evaporate first, leaving the other molecules in a gaseous state. This vapor is collected in another container by condensation and is known as distillate. The distillate is then separated from its nonvolatile companion by further chemical separation techniques. The process of distillation is used to purify many types of products including alcoholic beverages, perfumes, and food flavorings. It is also employed in industrial processes such as oil stabilization and air purification.

Some compounds cannot boil under atmospheric pressure, so the vaporization process must be carried out under lower pressure conditions. This is referred to as vacuum distillation and is usually performed using a rotary evaporator. This type of distillation is useful for compounds that would decompose if boiled under atmospheric pressure.

The vapor-liquid equilibrium of the liquid mixture in the distillation flask can be analyzed by plotting a thermodynamic curve on a diagram. This curve shows the temperature of the liquid in the flask and its composition as a function of time during the distillation. This information can help determine when to stop the distillation to obtain a pure product. A fractional distillation column can be inserted in the distillation setup to improve the separation by increasing the surface area on which vapors and condensates contact each other.

Vapor Density

The vapor density of a substance is its mass per volume in the gas phase divided by the mass of an equal volume of hydrogen. Vapor density has important implications when assessing whether a dangerous vapor can dissipate or travel long distances and should be included in section 9 of the safety data sheet.

Vapor density can also influence decisions regarding the use of specialized ventilation equipment when dealing with a chemical spill or leak. The lower the vapor density, the more likely the vapor will dissipate, making it less hazardous for personnel to work with.

A distillation process is a useful technique for separating mixtures of chemicals that have different boiling points. It is a common method for the separation of alcoholic beverages from their nonvolatile solids and for processing such industrial chemicals as formaldehyde and phenol. Distillation is also used in the production of automobile fuel and the purification of water.

A basic example of distillation is boiling a mixture of chemicals in a flask. At a certain temperature, the component with the lowest boiling point will vaporize first and be separated from the other liquids. A more advanced form of distillation is fractional distillation. This is a multi-step process that utilizes a fractionating column that improves separation by providing a large surface area for vapors and condensates to interact with one another.


Distillation is used to separate a mixture of substances into a pure compound, in both laboratory and industrial settings. It works on the principle that different substances have varying boiling points, so the most volatile component will evaporate first, leaving behind the less-volatile components in the liquid form. The vapor will then be condensed into a liquid again, yielding the desired distillate. This process can be repeated many times to achieve greater purity. The devices used for the vapor-condensation cycle are called distillation apparatus, and they can be of various designs. For example, a common laboratory setup for batch distillation is the Dean-Stark apparatus, shown below.

In practice, a distillation process is often conducted in a continuous mode, where the source material and vapors are continuously fed to the apparatus while the separation process is ongoing. Using this technique allows for a greater level of control over the distillation process, and it is used in such applications as oil refining.

The vapor-liquid equilibrium in a distillation apparatus can be improved by adding a column to the system that improves surface area and thus helps retain contact between the vapors and the liquid. Such a tower is often made up of a series of small subsystems (often called trays or dishes) that are each in a state of vapor-liquid equilibrium. A model is available to calculate the number of trays required for a certain degree of separation.