Taking mixtures apart
Because the parts of a mixture aren’t chemically bonded, they can be separated using physical methods. Each separation technique uses a difference in the parts’ properties, such as size, magnetism, solubility, density or boiling point.
Choosing a method
| Property that differs | Method | Example |
|---|---|---|
| Size of particles | Sieving | Sand from gravel |
| Magnetism | Magnetism (using a magnet) | Iron filings from sand |
| Dissolves or not | Filtration | Sand from water |
| Dissolved solid in liquid | Evaporation | Salt from salt water |
| Boiling point | Distillation | Pure water from salt water; alcohol from water |
| Density | Decanting, separating funnel or centrifuge | Oil from water |
| How far substances travel | Chromatography | Dyes in ink |
Sieving
A sieve has holes that let small particles fall through while catching larger ones. Gardeners sieve soil to remove stones. Gold panners swirl gravel in pans to separate heavy gold from lighter sand.
Magnetic separation
A magnet attracts iron, nickel and cobalt. Recycling centres use giant magnets to pull steel cans out of mixed waste.
Filtration
Filter paper has tiny holes that let liquid through but trap insoluble solids.
- The liquid that passes through is the filtrate.
- The solid left behind is the residue.
Examples: coffee filters, water treatment plants, furnace filters, pool filters. Filtration can’t separate a dissolved solid like salt from water, because dissolved particles are too small.
Decanting and settling
In a muddy mixture, heavy particles slowly sink. Decanting means carefully pouring off the liquid, leaving the solid behind. A separating funnel separates liquids that don’t mix, like oil and water: the denser liquid drains out the bottom first.
A centrifuge spins mixtures very fast, forcing denser parts to the bottom. Hospitals use centrifuges to separate blood into plasma and cells. Dairies use them to separate cream from milk.
Evaporation
Heating a solution evaporates the liquid, leaving the dissolved solid behind. Sea salt is made this way in shallow ponds that let the sun evaporate seawater. The downside: the liquid is lost.
Crystallization
If you evaporate part of a solution and let it cool slowly, crystals of the solute form. This produces pure crystals, like sugar or copper sulfate.
Distillation
Distillation separates liquids or recovers the liquid from a solution, using different boiling points.
- Heat the mixture. The substance with the lower boiling point evaporates first.
- The vapour travels into a condenser, a tube cooled by cold water.
- The vapour condenses back into a liquid, called the distillate, and is collected.
Uses:
- making pure (distilled) water
- desalination: turning seawater into drinking water
- separating crude oil into gasoline, diesel, kerosene and other fuels (fractional distillation)
- making perfumes and essential oils
Chromatography
Chromatography separates mixtures of dissolved substances, like dyes or pigments.
- Put a dot of ink near the bottom of a strip of paper.
- Dip the bottom edge in water (keep the dot above the water).
- Water moves up the paper and carries the dyes with it.
- Different dyes travel different distances, separating into bands of colour.
Black marker ink often separates into blue, purple, yellow and red! Forensic scientists use chromatography to analyse inks, drugs and poisons. Scientists also use it to test food and water.
Separating complex mixtures
Real mixtures often need several steps. To separate sand, salt and iron filings:
- Use a magnet to remove the iron filings.
- Add water to dissolve the salt.
- Filter to collect the sand.
- Evaporate the water to get the salt.
Common mistakes
- Using filtration for dissolved substances.
- Thinking distillation destroys the liquid. It collects it.
- Letting the ink dot sit in the water in chromatography; it just dissolves away.
Try it yourself
- Do paper chromatography with black and brown washable markers on a coffee filter. Compare the colours.
- Separate a mixture of sand, salt, rice and paper clips. Write your step-by-step plan first.
- Make a solar still: put salt water in a bowl with a cup in the centre, cover with plastic wrap and a pebble on top, and leave it in the sun.
Quick recap
- Mixtures are separated by physical methods that use differences in properties.
- Sieving and magnetism separate by size and magnetism.
- Filtration removes insoluble solids; evaporation recovers dissolved solids.
- Distillation separates by boiling point.
- Chromatography separates dissolved substances like dyes.