What Is Distillation?
Distillation is a separation technique that uses differences in boiling points to separate liquids from solutions or to separate mixtures of liquids. The desired liquid is evaporated by heating, then the vapour is cooled so it condenses back to a pure liquid and is collected.
Distillation is used when:
- You want to recover the solvent (not the solute).
- You want to separate two liquids with different boiling points.
Simple Distillation
Simple distillation is used to separate a dissolved solid from its liquid solvent (e.g. to get pure water from salt water).
Laboratory Setup:
- Heat the solution in a flask — the solvent (water) evaporates while the solute (salt) remains behind.
- The vapour travels along a tube into a condenser (a cool, jacketed glass tube with cold water flowing around it).
- In the condenser, the vapour cools and condenses back to liquid.
- The pure liquid (distillate) drips into a collection flask.
A thermometer placed in the distillation flask shows the temperature of the vapour. For simple distillation of water from salt water, the thermometer reads 100 °C throughout, because only water vapour is passing over. If the temperature rises above 100 °C, this indicates another liquid is beginning to evaporate.
Fractional Distillation
Fractional distillation separates a mixture of liquids with different boiling points, such as alcohol and water or the components of crude oil.
A fractionating column (a tall column packed with glass beads or plates) is placed above the flask. Liquids with lower boiling points rise higher in the column and condense at the top, while higher boiling point liquids condense lower down and drain back. Liquids are collected separately at different heights.
Example: Separating ethanol (boiling point 78 °C) from water (100 °C):
- At 78 °C, ethanol vapour rises and is collected.
- Water remains in the flask and is collected later.
Real-World Uses of Distillation
| Use | What is distilled |
|---|---|
| Drinking water | Seawater/contaminated water purified by distillation |
| Crude oil refining | Petroleum separated into petrol, diesel, kerosene, etc. |
| Perfume/essential oils | Fragrant compounds extracted from plants |
| Alcoholic beverages | Spirits (whisky, vodka) concentrated by distillation |
| Liquid air | Air cooled to liquid, then fractionated to get pure N₂, O₂, Ar |
You can purify water by a simple form of distillation using a camping stainless steel pot with the lid on — put muddy water inside, heat it gently, and place a cold wet cloth on the lid. Steam condenses on the inside of the cool lid as pure water drops and can be collected from the edge. This is why distilled water is used in batteries and irons — tap water contains dissolved minerals that would leave deposits.
Real world: Crude oil refinery — fractional distillation at industrial scale
An oil refinery is essentially a massive fractional distillation column, up to 60 metres tall. Crude oil (a complex mixture of hydrocarbons with different boiling points) is heated to ~350 °C at the bottom. Different fractions condense at different heights: bitumen at the bottom (highest boiling point), then heavy fuel oil, diesel, kerosene (for jet fuel), petrol, and gases at the top (lowest boiling points). Without distillation, crude oil would be useless — fractional distillation separates it into the fuels that power the modern world.
Real world: Whisky and vodka — distillation concentrates alcohol
When grain or fruit ferments, the resulting liquid contains 5–15% alcohol. To make spirits like whisky or vodka (40%+ alcohol), the liquid is distilled. Ethanol boils at 78 °C, water at 100 °C. Heating the mixture evaporates the alcohol first, which is collected and condensed. The Scottish whisky industry and Russian vodka tradition are both built on fractional distillation. Legally, a spirit must be distilled — hence the term "distillery." This is the same chemistry as a school fractional distillation experiment, just on a larger scale.
Real world: Desalination — making drinking water from seawater
Countries like Saudi Arabia, the UAE, and Kuwait get most of their fresh water from desalination — distilling seawater. Seawater is heated until it evaporates, the pure water vapour is cooled and condensed, and the salts stay behind. Modern "multi-stage flash" desalination plants process billions of litres per day. Alternatively, reverse osmosis (pushing water through membranes under pressure) separates water from dissolved salts. As freshwater scarcity grows globally, distillation is increasingly critical for human survival.
Where do you see this in real life? Petrol and diesel refined from crude oil (fractional distillation), whisky and vodka concentrating alcohol (ethanol boils at 78 °C, water at 100 °C), desalination plants converting seawater to drinking water, distilled water used in car batteries and steam irons (no mineral deposits) — distillation separates liquids by boiling point wherever purity matters.
Practice Problems
Why is distillation used to get pure water from sea water, rather than evaporation?
Show answer
A liquid mixture of water (boiling point 100 °C) and propanol (boiling point 97 °C) is distilled. Why is fractional distillation needed rather than simple distillation?