Separation Techniques – JSS2 Intermediate Science Lesson Note (First Term)

Separation techniques are physical methods used to separate the substances in a mixture.

Behavioural objectives

1. Define a mixture and give reasons for separating mixtures.
2. Describe filtration, evaporation, distillation and chromatography.
3. Use the terms filtrate, residue, solvent and distilled water correctly.
4. Choose the correct technique for a given mixture.
5. Give everyday Nigerian examples of separation methods.

What is a mixture and why do we separate it?

A mixture is a combination of two or more substances that are not joined chemically.

• Examples: salt in water, sand in garri, ink in water.
• Physical separation: the substances are not chemically joined, so physical methods can pull them apart.
• To get pure substances: for example, salt from seawater.
• To remove dirt and impurities: for example, cleaning muddy water.
• Everyday comparison: sorting laundry, where you separate whites from coloured clothes.

What is filtration?

Filtration is the method used to separate an insoluble solid from a liquid.

• Insoluble solid: a solid that does not dissolve in the liquid, such as sand in water.
• How it works: filter paper has tiny holes. The liquid passes through and the solid pieces are too big, so they are trapped.
• Filtrate: the clear liquid that drips through the filter paper.
• Residue: the solid left on the filter paper.
• Real-life example: using a sieve to remove chaff from pap (akamu), or filtering dirt out of muddy water.

Steps in filtration

1. Fold the filter paper and place it in a funnel.
2. Place the funnel over a clean beaker or flask.
3. Pour the mixture slowly into the funnel.
4. The filtrate collects in the beaker.
5. The residue stays on the filter paper.

What is evaporation?

Evaporation is the method used to get back a dissolved solid from a liquid by heating off the liquid.

• Why filtration fails: a dissolved solid, like salt in water, is too small and passes through the filter paper.
• How it works: the mixture is heated. The liquid turns to vapour and escapes into the air. The solid is left in the dish.
• The catch: the liquid is lost. If you evaporate salt water, you get the salt but the water is gone.
• Real-life example: salt from seawater. Seawater is put in large shallow pans under the sun. The sun evaporates the water and the salt is collected.

Steps in evaporation

1. Pour the salt solution into an evaporating dish.
2. Heat the dish gently.
3. The water turns to vapour and leaves the dish.
4. Salt crystals remain in the dish.

What is distillation?

Distillation is the method of separating a liquid from a solution by boiling it and then cooling the vapour back into liquid.

• Why it is used: it lets you collect the liquid, which evaporation loses. A person with only salty water needs the water to drink.
• Liebig condenser: a tube kept cold by water around it, which turns steam back into liquid.
• Distilled water: the pure water collected after distillation.
• Result: the salt stays in the first flask. The distilled water collects in the second flask.
• Two liquids: liquids with different boiling points, such as alcohol and water, can also be separated. The liquid with the lower boiling point boils off first.

Steps in distillation

1. The mixture is boiled in a flask.
2. The liquid turns to steam.
3. The steam travels through the Liebig condenser.
4. The cold tube turns the steam back into liquid drops.
5. The drops drip into a fresh container.

What is chromatography?

Chromatography is the method used to separate a mixture of colours or pigments, such as the dyes in ink.

• Example: black ink is often a mixture of dyes such as blue, red and purple.
• Solvent: the liquid, such as water or alcohol, that soaks up the paper and carries the colours.
• How it works: some colours travel fast and some travel slowly. Because they move at different speeds, they spread out into separate colours.
• Importance: it is used in medicine to test blood or urine for drugs, and by police to identify inks or chemicals.

Steps in paper chromatography

1. Draw a pencil line near the bottom of a strip of chromatography paper.
2. Put a small spot of ink on the line.
3. Dip the bottom edge of the paper in the solvent. The spot must stay above the solvent.
4. The solvent rises up the paper and carries the colours with it.
5. Remove the paper when the solvent is near the top and let it dry.

What other methods can separate mixtures?

Other simple methods include decantation, sieving, winnowing and magnetic separation.

• Decantation: pouring off the liquid after a heavy solid has settled, as with sand in water.
• Sieving: using a mesh to separate big particles from small ones, as with stones from sand or chaff from pap.
• Winnowing: using wind to blow away light chaff from heavier grains, as with rice or beans.
• Magnetic separation: using a magnet to pull out iron or steel from a mixture, as with iron filings from sand.

Which technique should be used for which mixture?

The correct technique depends on the substances in the mixture.

How can sand be removed from sugar?

Sand is insoluble and sugar dissolves in water, so you dissolve the sugar, filter, then evaporate.

1. Add water to the mixture and stir until the sugar dissolves.
2. Filter the mixture. The sand is the residue.
3. Heat the filtrate (sugar solution) to evaporate the water.
4. The sugar is left in the dish.

Evaluation

1. What is the name of the liquid that passes through the filter paper?
2. Why is evaporation not suitable when you need to keep the water from salty water?
3. Why do we use a condenser in distillation?
4. Name the method used to separate the dyes in black ink.
5. If sand falls into your sugar, how would you get the sugar back?

Assignment

A mixture contains sand, salt and water. Write the steps, in order, to get back the sand and the salt, and name the technique used at each step.