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Chemistry · Chapter 2: Separation and Purification
Simple and Fractional Distillation
Simple distillation to get pure water from salt water, and fractional distillation to separate ethanol and water and crude oil.
Key termRememberExampleWatch out
01Concept
Distillation is the process of heating a liquid to turn it into vapour, then cooling the vapour to get the liquid back. The liquid collected is the distillate.
Simple distillation
Simple distillation is used to separate a solvent from a solution, or a liquid from dissolved solids. Pure water from salt water.
- Heat the salt water in a flask. The water boils and becomes steam; the salt does not.
- The thermometer bulb is placed at the side arm, so it measures the temperature of the vapour leaving the flask (100 °C for water).
- The steam passes into the Liebig condenser, where cold water flowing around it condenses the steam into liquid.
- Pure water (the distillate) collects in the receiving flask. Salt stays behind in the flask.
Fractional distillation
Fractional distillation is used to separate miscible liquids with different boiling points. Ethanol (boiling point 78 °C) from water (100 °C).
- Heat the mixture. Both liquids evaporate, but the vapour is richer in ethanol, which has the lower boiling point.
- In the fractionating column, the vapour condenses and boils again many times on the glass beads. Each time, it gets richer in ethanol.
- Ethanol reaches the top first. The thermometer stays near 78 °C while ethanol distils over.
- When nearly all the ethanol has gone, the temperature rises to 100 °C and water starts to distil. Change the receiving flask to collect it separately.
| Simple distillation | Fractional distillation | |
|---|---|---|
| Separates | A liquid from dissolved solids | Two or more miscible liquids |
| Boiling points | Only the solvent evaporates | Used when the boiling points are close together |
| Extra apparatus | None | Fractionating column |
| Example | Pure water from seawater | Ethanol from water; crude oil into petrol, diesel, kerosene |
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Oil refineries
Refineries such as those in Karachi and Attock heat crude oil and separate it in tall fractionating towers into LPG, petrol, kerosene, diesel and bitumen.
Distilled water
Distilled water, made by simple distillation, is used in car batteries, steam irons and laboratories because it has no dissolved salts.
Desalination
Some coastal plants turn seawater into drinking water by distillation or by forcing it through membranes (reverse osmosis).
Rose water
Arq-e-gulab (rose water) is made by distilling rose petals with water; the fragrant oils come over with the steam.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What is distillation? What is the distillate?Show answer
Answer
Distillation is heating a liquid to vapour and condensing the vapour back to a liquid. The liquid collected is the distillate.
Q2.Why is the thermometer bulb placed at the side arm of the flask?Show answer
Answer
To measure the temperature of the vapour that is passing into the condenser, which is the boiling point of the liquid being collected.
Q3.Why does cooling water enter the condenser at the bottom?Show answer
Answer
So that the condenser stays completely full of cold water, giving the best cooling of the vapour.
Q4.Give one use each of simple and fractional distillation.Show answer
Answer
- Simple: getting pure (distilled) water from tap water or seawater.
- Fractional: separating crude oil into petrol, kerosene and diesel; separating ethanol from water.
Q5.A mixture of ethanol and water is fractionally distilled. Describe how the thermometer reading changes.Show answer
Answer
It rises to about 78 °C and stays there while ethanol distils. When the ethanol is used up, it rises to 100 °C and water distils.
Long questions
Q6.Describe, with a labelled diagram, how pure water can be obtained from salt water.Show answer
Answer
Diagram: round-bottom flask of salt water over a burner; thermometer with its bulb at the side arm; Liebig condenser with cold water in at the bottom and out at the top; receiving flask.
- Heat the salt water with a few anti-bumping granules.
- Water boils at 100 °C and changes into steam; the salt does not evaporate.
- The steam is cooled in the condenser and condenses to liquid water.
- Pure water (distillate) collects in the receiving flask; salt is left in the flask.