Simple and fractional distillation
Recover a solvent or separate miscible liquids by vaporisation, condensation and repeated enrichment in a fractionating column.
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Evaporation can recover a dissolved solid, but the solvent escapes. Distillation recovers a liquid by turning it into vapour and then cooling that vapour back into liquid. The collected liquid is the distillate.
If you need to decide between filtration, crystallisation and distillation, begin with choosing separation methods.
Simple distillation: recover a solvent
Use simple distillation to recover a volatile solvent from a solution containing a non-volatile solute. For example, water can be collected from salt solution while the dissolved salt remains in the flask.
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- Heat the solution using the instructed apparatus. The solvent vaporises; the non-volatile solute does not travel with it as vapour.
- Vapour passes the thermometer bulb at the side arm and enters the condenser.
- Cooling water flowing through the outer jacket removes energy. Vapour in the inner tube condenses and runs into the receiver.
Cooling water enters the lower connector and leaves the upper connector, filling the jacket and displacing air. It stays separate from the distillate. The thermometer measures the outgoing vapour, so its bulb belongs at the side arm, rather than in the liquid.
A supervised distillation setup must have an outlet: do not heat liquid in a sealed apparatus. Follow the instructions for safe heating and stop before boiling the flask dry.
Fractional distillation: improve separation of miscible liquids
Miscible liquids form one liquid phase, so a separating funnel cannot separate them. Fractional distillation uses differences in boiling behaviour. It is particularly useful when the boiling points are relatively close and simple distillation gives poor separation.
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The fractionating column adds repeated vaporisation and condensation between the flask and condenser. At each stage, the vapour becomes richer in the more volatile component, usually the liquid with the lower boiling point. The less volatile component tends to condense and run back down the column. The first collected fraction is therefore richer in the more volatile liquid.
For example, pure ethanol and pure water have approximate boiling points of 78°C and 100°C at atmospheric pressure. Fractional distillation gives an ethanol-rich early fraction from their mixture. Water can also be present in the vapour; it does not wait until 100°C before any of it vaporises. Distillation improves separation but does not guarantee that each collected fraction is completely pure.
Ethanol and many organic solvents produce flammable vapours. Use the supervised heating method, keep vapours away from naked flames and wear eye protection.
Choose from the mixture and the goal
| Goal | Suitable method | Reason |
|---|---|---|
| Recover water from salt solution | Simple distillation | Water vaporises and condenses; the non-volatile salt remains. |
| Obtain salt from salt solution | Evaporation or crystallisation | The solid is the wanted product; collecting the solvent is unnecessary. |
| Improve separation of ethanol and water | Fractional distillation | Repeated vaporisation and condensation enrich the more volatile component. |
| Separate oil and water | Separating funnel | These immiscible liquids form layers; heating is unnecessary. |
Worked example
Examiner practice 1
Choosing Simple vs Fractional Distillation
Examination question
Name the method and property reason
View solution step by step
Choose the method
1 markMethod
Select fractional distillation.Reason
A fractionating column provides repeated evaporation and condensation.Working
Method: fractional distillation.State the property reason
1 markMethod
Identify ethanol and water as miscible liquids with relatively close boiling points.Reason
Simple distillation would give poorer separation when both vapours can be present over a close temperature range.Working
Fractional distillation gives better separation of the two miscible liquids.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the method and boiling-point/miscibility reason.
Try it independently
Mind stretcher 1: Two water paths, two different jobsExtension
A learner says that the condenser’s cooling water flows into the receiver and becomes the distillate. Correct the explanation and state why cooling water enters through the lower connector.
Show answer
Cooling water flows through the outer jacket; solvent vapour flows through the separate inner tube. Energy passes from vapour to the cooler jacket, so the vapour condenses and the distillate enters the receiver. Feeding water into the lower connector fills the jacket and displaces air through the upper connector.
Mind stretcher 2: Choose which product to keepExtension
Two groups have salt solution. Group A wants dry salt; Group B wants water. Choose a method for each and explain where the wanted substance ends up.
Show answer
Group A can evaporate the water to obtain the heat-stable salt, or use crystallisation if crystals are wanted. Group B uses simple distillation: water vaporises and condenses into the receiver while the salt remains in the flask. Filtration cannot remove dissolved salt ions from water.
Practise and check
See what you know across this topic, then go back to anything you got wrong.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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