Lesson focus: distillationSeparating a liquid from a mixture by boiling it and then cooling the gas back into a liquid. separates substances through vaporisation and condensation; its success depends on volatility and boiling-point differences.
Compare simple and fractional distillation and explain each apparatus component.
Build on earlier learning
Evaporation and distillationYear 7 introduced evaporation and distillation. This lesson adds vapour composition, fractionation and apparatus reasoning.
Practical
Simple distillation
- Heat salty water in a flask fitted with a thermometer and a condenser.
- Water boils, the vapour condenses in the cold condenser, and pure water collects.
- The thermometer at the side arm reads the boiling point of the distillate.
Record: The boiling point and observations of what is left behind.
Safety: Add anti-bumping granules; do not heat a sealed flask.
You will be successful when you can…
- Explain simple distillation.
- Explain condenser flow.
- Compare fractional distillation.
- Identify safety and purity limits.
Words that carry the science
- distillation
- Separation using vaporisation followed by condensation.
- distillate
- Condensed liquid collected after distillation.
- volatile
- Able to vaporise relatively readily.
- condenser
- A cooled tube where vapour changes to liquid.
- fractionating column
- A column providing repeated vaporisation and condensation.
- boiling point
- Temperature at which vapour pressure equals external pressure.
Simple distillation recovers a volatile component
01Simple distillation recovers a volatile component
Heat a solution so the more volatileTurns into a gas easily, like perfume. component vaporises. Vapour enters a water-cooled condenserA cooled glass tube that turns gas back into liquid during distillation., loses energy and condenses into a receiving vessel as the distillateThe pure liquid collected at the end of distillation..
For salt solution, water distils while non-volatile salt remains. The thermometer bulb should measure vapour entering the condenser, not the hot liquid or room air.
02Cooling water travels upward
Cooling water enters the condenser at the lower port and leaves at the upper port. This fills the jacket completely and maintains counter-current cooling.
The apparatus must never be sealed: heating a closed system can produce dangerous pressure. Anti-bumping granules are added before heating to promote smooth boiling.
03Fractional distillation improves close-boiling separation
A fractionating columnA tall column used to separate a mixture of liquids with different boiling points. creates many condensation–vaporisation steps. Rising vapour becomes richer in the lower-boiling component before reaching the condenser.
Separation is less effective when boiling pointThe temperature at which a liquid boils, such as 100 °C for pure water. are close, the column is short or collection is too rapid. Fractions are collected over temperature ranges and may still need purity checks.
Simple distillation
Heat the mixture: the water boils, the steam cools and condenses in the condenser, and pure water drips into the beaker. The salt or dye is left behind.
Steam (gas) Water (liquid)
Controls
Rates are illustrative. The thermometer bulb is level with the side arm, so it reads the temperature of the steam going into the condenser.
Choose a distillation method
Separate water from sodium chloride solution, then consider ethanol and water.
- Salt is non-volatile under the conditions, so simple distillationSeparating a liquid from a mixture by boiling it and then cooling the gas back into a liquid. can recover water.
- Water vapour condenses and is collected; salt remains.
- Ethanol and water are both volatile liquids.
- Use fractional distillation because repeated equilibration improves their separation.
How to make the reasoning visible: Method choice depends on component volatility, not whether the starting mixture looks uniform.
Distillation investigation
Interpret a model collection record from a liquid mixture.
| Collection interval / °C | Volume / cm³ | Likely fraction | Purity comment |
|---|---|---|---|
| 76–80 | 12 | lower-boiling rich | temperature fairly stable |
| 80–90 | 4 | mixed transition | do not combine automatically |
| 96–100 | 18 | higher-boiling rich | second plateau |
| above 100 | 0 | stop heating | avoid dryness |
Why collect the 80–90 °C liquid separately?
Why stop before the flask is dry?
Check your understanding
Answer from memory, then check against the suggested answer.
What is distillate?
Where should cooling water enter?
Why use a fractionating column?
Why is desalinated distillate not guaranteed drinkable from an unknown water sample?
Why does reducing pressure lower boiling point?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Fractional distillation separates two miscible liquids with different boiling points.
Describe what happens when the mixture is heated.
(2)Explain how the fractionating column separates the liquids.
(4)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., describe what happens to vapour composition inside the column, not merely the apparatus names.