The Art of Distillation: Separating Liquids
The Core Principles: Why Boiling and Condensing Work
To understand distillation, we must first understand what happens when a liquid is heated. Every pure substance has a specific temperature at which it changes from a liquid to a gas; this is its boiling point (B.P.)¹. For example, pure water boils at 100°C (or 212°F) at sea level, while ethanol (drinking alcohol) boils at a lower temperature of about 78°C (172°F).
When you have a solution—like saltwater or a mixture of alcohol and water—the components have different boiling points. When heat is applied, the component with the lower boiling point (the more "volatile" one) will turn into vapor more easily. This vapor is then led away from the original mixture through a tube. By cooling this vapor in a separate chamber (the condenser), it loses energy, and the gas molecules slow down and come together to form a liquid again. This new liquid, called the distillate, is much purer in the component that boiled first.
The Anatomy of a Still: A Step-by-Step Walkthrough
The classic apparatus for distillation is called a still. Whether it's a simple lab setup or a giant industrial column, all stills have four main components.
| Component | Function | Real-World Analogy |
|---|---|---|
| Distillation Flask | Holds the original mixture to be heated. Also called the "pot." | A kettle on a stove, holding the water you're about to boil. |
| Heat Source | Applies controlled heat to the flask, causing vaporization. | The stove burner under the kettle. |
| Condenser | A cooled tube where the hot vapor loses energy and turns back into liquid. | The cold lid of a pot where steam turns into water droplets. |
| Collection Flask | Collects the purified liquid (distillate) that drips from the condenser. | A cup placed to catch the droplets falling from the pot lid. |
The process flows in one direction: Heat → Vaporize → Cool → Condense → Collect. In a lab, the condenser is often a "Liebig condenser," which has an outer jacket with cold water constantly flowing through it to cool the inner tube carrying the vapor.
Simple vs. Fractional: Choosing the Right Tool
Not all mixtures are created equal, so chemists use different types of distillation. The two most common are Simple Distillation and Fractional Distillation.
Simple Distillation is best for mixtures where the boiling points of the components are very different (e.g., more than 25°C apart). It's perfect for separating salt from seawater or purifying water that contains non-volatile impurities. The setup is straightforward, as described in the table above. However, if the boiling points are close, simple distillation will produce an impure distillate, as both components will vaporize to some degree.
Fractional Distillation is the solution for close-boiling mixtures, like ethanol and water or the various components of crude oil. The key difference is the addition of a fractionating column between the distillation flask and the condenser. This column is filled with material (like glass beads or metal plates) that provides a large surface area. As vapor rises, it repeatedly condenses and re-vaporizes on this material. With each cycle, the vapor becomes richer in the lower-boiling component. Think of it as many simple distillations happening one after the other in a single column. The different purified components, called "fractions," are collected at different temperatures.
From Science Lab to Everyday Life: Distillation in Action
Distillation is not just a lab experiment; it's a process that touches our daily lives in countless ways.
Water Purification: In regions without clean tap water, simple solar stills can be life-saving. Dirty or salty water is heated by the sun, evaporates, condenses on a cool surface (like plastic sheeting), and drips into a clean container, leaving behind salts, bacteria, and other contaminants. On a larger scale, desalination plants use massive distillation units to turn seawater into fresh drinking water.
Food and Beverages: The production of alcoholic spirits like whiskey, vodka, and gin relies entirely on distillation. Fermented mash, which contains ethanol and water, is heated in a still. Since ethanol boils first, its vapor is collected and condensed, resulting in a much higher alcohol content. Similarly, many of the essential oils that give perfumes and food flavorings their scent—like lavender oil or peppermint oil—are extracted from plants using steam distillation.
Fueling Our World: Perhaps the most impactful application is in an oil refinery. Crude oil is a complex mixture of hydrocarbons³. It is useless in its raw form. In giant fractionating towers, the crude oil is heated, and its vapors rise through many levels (trays). Different fractions condense at different heights based on their boiling points: gases (like propane) at the top, then gasoline, kerosene (jet fuel), diesel, and heavy lubricating oils and asphalt at the bottom. This single process provides the fuels that power transportation and industry.
Important Questions Answered
Can distillation produce 100% pure alcohol from water?
Is distilled water safe to drink?
What is the main energy cost in distillation?
Footnote
¹ Boiling Point (B.P.): The temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, causing the liquid to change into a vapor throughout.
² Raoult's Law: A law of physical chemistry stating that the partial vapor pressure of each component in an ideal mixture of liquids is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture.
³ Hydrocarbons: Organic compounds consisting entirely of hydrogen and carbon atoms. They are the main components of fossil fuels like petroleum and natural gas.
⁴ Azeotrope: A mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation. This happens because when an azeotrope is boiled, the vapor has the same proportions of constituents as the unboiled mixture.