Simple and Fractional Distillation
Distillation is a method of separating liquids by heating a mixture to its boiling point, collecting the vapor and condensing it back into a liquid; simple distillation separates a liquid from a dissolved solid, and fractional distillation separates liquids with different boiling points. In brief: simple distillation turns seawater into fresh water, and fractional distillation separates crude oil into petrol, kerosene and diesel.
Distillation uses the fact that different substances boil at different temperatures. The substance with the lower boiling point vaporizes first. This article explains how simple and fractional distillation work, describes the apparatus, compares the two methods, includes the boiling points of common liquids, works through an example and describes the history and everyday uses.
What is simple distillation?
The mixture is heated in a flask. The liquid with the lowest boiling point turns to vapor, which travels into a condenser. The condenser is a tube cooled by flowing cold water in a surrounding jacket, which turns the vapor back into liquid. The liquid, called the distillate, drips into a receiving flask. The solid or the liquid with the higher boiling point stays in the original flask.
Simple distillation works best when the components have very different boiling points (a difference of about 70 °C or more) or when a liquid is separated from a non-volatile solid, such as water from dissolved salt.
What is fractional distillation?
When the liquids have boiling points that are closer, simple distillation gives a distillate that still contains both. Fractional distillation adds a fractionating column between the flask and the condenser. The column is filled with glass beads or has plates that provide a large surface. As the vapor rises, it repeatedly condenses and re-evaporates on the surface. Each cycle enriches the vapor in the more volatile (lower boiling) component.
At the top of the column the temperature is close to the boiling point of the most volatile component, and that component distills first. A thermometer at the top of the column shows which component is being collected. The temperature rises when the first component is nearly gone, and the next fraction can be collected in a different flask.
Simple and fractional distillation compared
| Feature | Simple distillation | Fractional distillation |
|---|---|---|
| Fractionating column | No | Yes |
| Separates | Liquid from solid, or liquids with very different boiling points | Liquids with closer boiling points |
| Purity of distillate | Moderate | Higher |
| Speed | Faster | Slower |
| Typical example | Making pure water from seawater | Separating ethanol from water, refining crude oil |
Boiling points of common liquids
| Liquid | Boiling point (°C) |
|---|---|
| Diethyl ether | 35 |
| Acetone | 56 |
| Methanol | 65 |
| Ethanol | 78 |
| Water | 100 |
Worked example: separating ethanol and water
A mixture of 100 mL of ethanol and 100 mL of water is separated by fractional distillation.
- Compare boiling points. Ethanol boils at 78 °C and water at 100 °C, a difference of 22 °C, which is too small for a good simple distillation.
- Heat gently. Use a water bath or a gentle heater. The thermometer at the top of the column rises to about 78 °C and stays there while ethanol-rich vapor distills.
- Collect the first fraction. Collect the distillate while the temperature stays close to 78 °C.
- Change the receiver. When the temperature starts to rise toward 100 °C, replace the receiving flask. Most of the remaining liquid is water.
Even the best fractional distillation of ethanol and water cannot go beyond about 95.6% ethanol by mass, because this mixture forms an azeotrope, a mixture that boils at constant composition. Other methods are needed to obtain absolute ethanol.
How is crude oil separated?
In an oil refinery, heated crude oil enters a tall fractionating tower that is hottest at the bottom and coolest at the top. Fractions with large molecules and high boiling points condense low in the tower, and fractions with small molecules and low boiling points rise to the top.
| Fraction | Approximate boiling range (°C) | Typical use |
|---|---|---|
| Refinery gases | below 40 | Heating gas, bottled gas |
| Gasoline (petrol) | 40 to 200 | Car fuel |
| Kerosene | 175 to 275 | Jet fuel |
| Diesel oil | 200 to 350 | Diesel engines |
| Fuel oil and bitumen | above 350 | Ships, road surfaces |
Boiling ranges differ slightly between sources, and the values above are approximate.
How did distillation develop?
Distillation has been used for many centuries. Early alchemists in the Middle East and Europe refined the apparatus, and the alembic, a type of still, became widely known. Perfume, alcoholic spirits and medicinal waters were produced by distillation. The industrial distillation of crude oil began in the 1850s, when kerosene was produced for lamps, and continuous fractional distillation later became the heart of the petroleum industry.
Which mistakes are common?
Mistake 1: Heating a closed apparatus
The system must be open to the air at the end of the condenser. A sealed system can explode as pressure builds up.
Mistake 2: Putting the thermometer bulb in the wrong place
The bulb must be level with the side arm that leads to the condenser, so it measures the temperature of the vapor, not of the boiling liquid.
Mistake 3: Running the cooling water in the wrong direction
Cold water must enter at the bottom of the condenser and leave at the top, which keeps the jacket full and cools efficiently.
Mistake 4: Boiling the flask to dryness
Never distill all the liquid. Stop when a small amount remains, because the residue may overheat or contain dangerous substances.