Standard Separation Techniques for Mixtures – SS1 Chemistry Lesson Note (Second Term)

Separation techniques are methods used to split a mixture into its pure constituents.

Behavioural objectives

By the end of the lesson, students should be able to:

1. state why mixtures are separated into their constituents;
2. name the standard techniques for separating mixtures;
3. explain how each technique works and give examples of where it is used;
4. choose a suitable technique for separating a given mixture;
5. describe how paper chromatography is carried out;
6. state the criteria for purity of a substance.

Why do we separate mixtures?

We separate mixtures because most substances are needed in their pure form.

• To get pure substances: pure salt, sugar and distilled water are made from mixtures.
• To remove harmful impurities: dirt and germs are removed from water before drinking.
• To recover useful materials: metals and fuels are taken from ores and crude oil.
• To analyse a sample: blood, drugs and dyes are separated and tested.

What decides the choice of separation technique?

The technique chosen depends on a physical property that is different in the substances present.

• Solubility: one substance dissolves in a solvent and the other does not.
• Particle size: one substance has larger particles than the other.
• Boiling point: the liquids boil at different temperatures.
• Density: one liquid is denser than the other.
• Magnetism: one substance is attracted to a magnet.
• Ability to sublime: one solid changes directly to gas on heating.

What are the standard separation techniques?

There are fourteen standard separation techniques: filtration, centrifugation, decantation, evaporation, crystallisation, fractional crystallisation, precipitation, sublimation, distillation, fractional distillation, use of a separating funnel, sieving, magnetic separation and chromatography.

Summary table of the techniques

1. What is filtration?

Filtration is the separation of an insoluble solid from a liquid using a filter.

• Residue: the solid left on the filter paper.
• Filtrate: the clear liquid that passes through.
• Example: chalk particles are separated from water.
• Uses: water purification plants and breweries.

2. What is centrifugation?

Centrifugation is the separation of insoluble solids from a liquid by spinning the mixture in a centrifuge.

• Centrifuge: a machine that spins test tubes containing suspensions at high speed.
• Best for: small amounts of material.
• Example: in hospitals, blood cells are separated from plasma.

3. What is decantation?

Decantation is the separation of an insoluble solid from a liquid by allowing the solid to settle and pouring off the liquid.

1. Allow the mixture to stand until the solid settles.
2. Pour the clear upper liquid carefully into a clean container.
3. Leave the solid in the original container.

• Limitation: it is quick but not accurate, because some liquid or solid is left behind.

4. What is evaporation?

Evaporation is the recovery of a soluble solid (solute) from its solvent by heating off the solvent.

• Example: salt is recovered from salt solution.
• Solar evaporation: seawater is fed into large ponds, and the sun and wind slowly evaporate the water.
• Collection: the salt is collected after the water has gone.

5. What is crystallisation?

Crystallisation is the formation of pure crystals from a solution, used for salts that decompose easily on heating.

1. Heat the solution to concentrate it.
2. Allow the solution to cool, so that crystals start to form.
3. Filter off the crystals and dry them.

To induce crystal formation:

• Seeding: add a crystal of the salt.
• Scratching: scratch the inside of the container with a glass rod.

• Uses: industries where purity matters, such as the drug and sugar industries.

6. What is fractional crystallisation?

Fractional crystallisation is the separation of two or more solutes present in the same solution.

• Condition: the solutes must have different solubilities at different temperatures.
• Method: as the solution cools, crystals of one solute appear first, leaving the other solutes dissolved.

7. What is precipitation?

Precipitation is the separation of a solid from solution by using its different solubility in two miscible liquids.

• Example: FeSO₄ dissolves in water but not in ethanol.
• Method: add ethanol to a solution of FeSO₄ in water.
• Result: the FeSO₄ is precipitated and can be filtered out.

8. What is sublimation?

Sublimation is the change of state from solid to gas directly on heating, without passing through the liquid state.

• Substances that sublime: iodine and ammonium chloride.
• Use: to separate these solids from other materials, such as a mixture of ammonium chloride and sodium chloride.
• Sublimate: the pure solid recovered when the vapour cools.

9. What is distillation?

Distillation is the recovery of a solvent from a solution by boiling it and condensing the vapour.

1. Heat the solution in the distillation flask.
2. The solvent boils and its vapour rises into the condenser.
3. Cold water in the condenser cools the vapour back to liquid.
4. The liquid, called the distillate, drips into a receiver.

• Left behind: the solute and other impurities stay in the flask.
• Uses: gin distilleries and the making of distilled water.

10. What is fractional distillation?

Fractional distillation is the separation of two or more miscible liquids based on their different boiling points.

• When it is used: when the boiling points of the liquids are close, simple distillation does not separate them well.
• Fractionating column: a column fitted to the distillation flask to improve the separation.
• Order of collection: the liquid with the lower boiling point distils first.

Uses of fractional distillation:

• separating crude oil into fractions;
• separating liquid air into oxygen and nitrogen;
• manufacturing spirits;
• separating a mixture of benzene and methylbenzene.

11. What is a separating funnel used for?

A separating funnel is used to separate immiscible liquids, which are liquids that do not mix.

1. Pour the mixture into the funnel and let it stand.
2. The less dense liquid forms the top layer and the denser liquid forms the bottom layer.
3. Open the tap and run off the lower liquid into a beaker.
4. Close the tap and collect the upper liquid in another container.

• Example: water and kerosene. Kerosene is less dense, so it floats on the water.

Diagram of a separating funnel

12. What is sieving?

Sieving is the separation of solid particles of different sizes using a sieve.

• Method: particles smaller than the mesh pass through and larger particles stay behind.
• Uses: gold and diamond mines, and garri production.

13. What is magnetic separation?

Magnetic separation is the removal of magnetic substances from non-magnetic ones using a magnet.

• Uses: the mining and steel industries.
• Example 1: removing magnetic impurities from tin ore.
• Example 2: separating iron filings from sulphur powder.

14. What is chromatography?

Chromatography is the separation of a mixture of solutes by a solvent moving over a porous adsorbent medium such as paper.

• Principle: the adsorbent holds the different constituents of a sample at different rates, so they separate.
• Use by chemists: to analyse a complex mixture of substances.

Applications of chromatography:

• measuring the level of pollutants in the air;
• analysing drugs;
• testing blood and urine samples;
• identifying petroleum fractions in industry;
• scientific research.

What are the types of chromatography?

There are four types of chromatography: paper, thin layer, gas and column.

• Paper chromatography: the solvent moves up a strip of paper.
• Thin layer chromatography: the solvent moves over a thin layer of adsorbent spread on a plate.
• Gas chromatography: a gas carries a vaporised sample through a column.
• Column chromatography: the mixture passes down a tube packed with adsorbent.

How is paper chromatography carried out?

1. Place a spot of the solution, such as ink, on the paper near one end.
2. Dip that end of the paper into a suitable solvent, such as water or ethanol, in a closed jar. The spot must stay above the solvent level.
3. Leave the solvent to move up the paper.
4. Remove the paper and dry it.
5. Read the spots. Each spot is a different substance in the ink or dye.

What are the criteria for purity?

A pure substance has a fixed melting point, a fixed boiling point and gives one spot on a paper chromatogram.

• Melting point: fixed for a pure substance. Impurities lower it.
• Boiling point: fixed for a pure substance. Impurities raise it.
• Chromatogram: a pure substance gives one spot only.

Evaluation

1. Which technique can be used to separate chalk particles from water?
2. Explain how salt is recovered from salt solution.
3. Why can a separating funnel be used to separate water and kerosene?
4. When is fractional distillation used instead of simple distillation?
5. Describe how paper chromatography is carried out.

Assignment

1. Describe, step by step, how you would separate a mixture of sand, salt and iron filings. Name the technique used at each step.