Mixtures and Separation
4. Distillation
Learning outcomes
- I can explain how distillation separates substances based on boiling point differences.
- I can describe the apparatus used in simple distillation.
- I can explain how pure liquids can be recovered through distillation.
- I can distinguish between simple and fractional distillation.
- I can apply distillation concepts to real-world examples such as water purification and petroleum refining.
Introduction
Some mixtures cannot be separated using filtration or evaporation because the substances are both liquids or because we want to recover the liquid rather than the dissolved solid. In these situations, chemists often use distillation, a separation technique that takes advantage of differences in boiling points.
Distillation is one of the oldest and most important laboratory techniques. It is used to produce clean drinking water, manufacture perfumes and medicines, separate alcoholic drinks, and refine crude oil into fuels such as petrol and diesel. By carefully heating a mixture, scientists can collect pure liquids for many different purposes.
What Is Distillation?
Distillation is a method of separating substances based on differences in their boiling points.
During distillation:
- The mixture is heated.
- The substance with the lower boiling point evaporates first.
- The vapour is cooled.
- The vapour condenses back into a liquid.
- The pure liquid is collected.
This process allows one liquid to be separated from another or recovered from a solution.
Figure 1. Distillation separates substances by heating, evaporating, cooling, and condensing a liquid.
The Principle Behind Distillation
Different substances boil at different temperatures.
For example:
| Substance. | Boiling Point |
|---|---|
| Ethanol | 78°C |
| Water | 100°C |
If a mixture of ethanol and water is heated:
- Ethanol reaches its boiling point first.
- Ethanol vapour forms before most of the water.
- The vapour is cooled and collected.
This difference in boiling points makes separation possible.
Simple Distillation Apparatus
A simple distillation setup contains:
- A heating source.
- A distillation flask.
- A thermometer.
- A condenser.
- Cooling water.
- A receiving flask or beaker.
The condenser cools the hot vapour so it changes back into a liquid.
The collected liquid is called the distillate.
Figure 2. The main parts of a simple distillation apparatus include a flask, thermometer, condenser, and receiving vessel.
How Simple Distillation Works
Suppose we want to obtain pure water from salt water.
Step 1
Heat the salt solution.
Step 2
Water reaches its boiling point.
Step 3
Water vapour rises into the condenser.
Step 4
Cold water flowing through the condenser cools the vapour.
Step 5
The vapour condenses into pure liquid water.
Step 6
The dissolved salt remains behind in the flask.
This allows the solvent to be recovered instead of being lost through evaporation.
Recovering Pure Liquids
Simple distillation is commonly used to recover:
- Pure water from seawater.
- Distilled water for laboratories.
- Water from contaminated sources.
- Some solvents used in chemical industries.
Unlike evaporation, distillation allows the liquid to be collected and reused.
Figure 3. Simple distillation recovers pure water while dissolved salts remain in the flask.
Simple vs Fractional Distillation
There are two main types of distillation.
Simple Distillation
Used when:
- One liquid is mixed with a dissolved solid.
- Two liquids have very different boiling points.
Examples:
- Salt water.
- Sugar solution.
Fractional Distillation
Used when:
- Two or more liquids have similar boiling points.
A fractionating column is placed between the flask and the condenser.
The column allows repeated evaporation and condensation, improving the separation.
Examples:
- Ethanol and water.
- Crude oil.
Figure 4. Fractional distillation uses a fractionating column to separate liquids with similar boiling points.
Comparing the Two Methods
| Simple Distillation | Fractional Distillation |
|---|---|
| No fractionating column | Uses a fractionating column |
| Separates liquids with large boiling point differences. | Separates liquids with similar boiling points |
| Suitable for recovering a solvent from a solution | Suitable for separating mixtures of liquids |
| Simpler apparatus | More complex apparatus |
Choosing the correct method depends on the substances being separated.
Real-World Applications
Water Purification
Seawater can be distilled to produce fresh drinking water.
This process is called desalination.
Petroleum Refining
Crude oil is separated by fractional distillation.
Products include:
- Petrol (gasoline)
- Kerosene
- Diesel
- Lubricating oils
- Bitumen
Each fraction is collected at a different temperature.
Food and Drink
Distillation is used to:
- Produce alcoholic beverages.
- Extract essential oils.
- Manufacture perfumes.
Laboratories
Scientists use distilled water because it contains very few dissolved impurities.
Figure 5. Distillation is widely used in water purification, petroleum refining, and many industrial processes.
Why Distillation Is Important
Distillation is one of the most valuable separation techniques because it allows scientists to:
- Recover pure liquids.
- Purify chemicals.
- Produce clean drinking water.
- Separate useful fuels.
- Manufacture medicines and industrial chemicals.
Many modern industries depend on distillation every day.
Worked Example
Question
Which separation method should be used?
| Mixture | Best Method |
|---|---|
| Salt water | Simple distillation |
| Ethanol and water | Fractional distillation |
| Crude oil | Fractional distillation |
| Copper sulfate solution (recover crystals only). | Evaporation |
Explanation
- Recovering a liquid from a solution → Simple distillation
- Separating two liquids with similar boiling points → Fractional distillation
- Recovering only the dissolved solid → Evaporation
Real-World Connection
Many countries with limited freshwater supplies use desalination plants to convert seawater into drinking water. One common method involves distillation, where seawater is heated until the water evaporates, leaving the dissolved salts behind. The water vapour is then condensed to produce clean, fresh water. Fractional distillation is also essential in oil refineries, where crude oil is separated into fuels and other useful products that power vehicles, heat homes, and provide raw materials for making plastics.
Did You Know?
The International Space Station (ISS) recycles much of its water to reduce the amount that must be launched from Earth. The station uses advanced purification systems that include processes similar to distillation and condensation to recover clean water for astronauts, making long-term space missions possible.
Key Terms
Boiling point – The temperature at which a liquid changes into a gas.
Condenser – A piece of apparatus that cools vapour so that it condenses back into a liquid.
Condensation – The process by which a gas changes into a liquid.
Distillate – The purified liquid collected during distillation.
Distillation – A separation technique that separates substances based on differences in boiling points.
Fractional distillation – A form of distillation that uses a fractionating column to separate liquids with similar boiling points.
Fractionating column – A column that allows repeated evaporation and condensation during fractional distillation.
Simple distillation – A method used to recover a liquid from a solution or separate liquids with very different boiling points.
Vapour – The gaseous form of a substance that is normally a liquid at room temperature.
Key Takeaways
- Distillation separates substances using differences in their boiling points.
- In simple distillation, the substance with the lower boiling point evaporates first, then condenses to form the distillate.
- A simple distillation apparatus includes a flask, thermometer, condenser, and receiving vessel.
- Simple distillation is used to recover a liquid from a solution or separate liquids with very different boiling points.
- Fractional distillation uses a fractionating column to separate liquids with similar boiling points.
- Distillation has many important applications, including water purification, petroleum refining, food production, and chemical manufacturing.