Separation techniques for mixtures Revision Notes | GCSE AQA Higher Chemistry

1. Why Separation Matters

You drink water from the tap, but it started as a mixture containing dirt, bacteria and dissolved chemicals. How did it become pure enough to drink? The answer lies in separation techniques - physical processes that split mixtures apart without creating new substances.

Unlike chemical reactions, these methods don't break or form bonds. They use the fact that different substances have different physical properties - size, boiling point, solubility. Think of it like sorting a mixed bag of sweets by colour or size.

2. Filtration - Size Matters

Picture trying to separate sand from water. The sand particles are much larger than water molecules, so they can't fit through the tiny holes in filter paper. The water passes through (called the filtrate), while the sand stays behind (called the residue).

This works because we're exploiting a physical difference - particle size. The filter paper acts like a molecular sieve, letting small particles through but blocking large ones.

3. Crystallisation - Evaporate to Separate

When salt dissolves in water, it seems to disappear. But it's still there - just broken down into tiny particles too small to see. To get it back, we need to remove the water.

Worked example: Crystallising Salt from Salt Water

Separate salt from a salt water solution using crystallisation

1. Use a Bunsen burner on a low flame to warm the salt water — Gentle heating prevents the solution from spitting and losing crystals
2. Continue heating until the solution becomes concentrated — Stop when you see the first crystals forming around the edges
3. Turn off heat and allow slow cooling at room temperature — Slow cooling produces larger, purer crystals than rapid cooling
4. Use filter paper to separate crystals from remaining solution — The remaining solution contains impurities - pure crystals are left behind

Answer: Pure salt crystals separated from the original mixture

4. Simple Distillation - Boiling Point Differences

Water boils at 100°C, but salt doesn't boil until over 1400°C. This huge difference lets us separate them completely. We heat the mixture until the water evaporates, then cool the vapour back into pure water.

The key equipment is a condenser - a tube surrounded by cold water. Hot water vapour enters, gets cooled down, and condenses back into liquid water. The salt stays behind in the original flask because it never reached its boiling point.

5. Fractional Distillation - Separating Similar Liquids

Simple distillation works when boiling points are very different (like water and salt). But what if you need to separate ethanol (boiling point 78°C) from water (100°C)? The 22°C difference is too small for simple distillation.

The secret weapon is a fractionating column - a tall tube packed with glass beads. As vapours rise up the column, they cool down. The liquid with the higher boiling point condenses and drips back down, while the lower boiling point liquid continues up to be collected.

This creates multiple separation stages in one apparatus. Each time vapour condenses and re-evaporates, the separation gets better. It's like having several simple distillation setups stacked on top of each other.

6. Chromatography - Separating by Movement

Imagine a race where different coloured inks have to climb up a piece of paper. Some inks are 'stickier' to the paper and move slowly. Others are less sticky and race ahead. This is exactly how paper chromatography works. The Rf value is calculated using Rf = distance moved by the substance divided by distance moved by the solvent front, and substances can be identified by comparing Rf values measured under the same conditions.

The paper is the stationary phase (it doesn't move), and the solvent (e.g. water or ethanol) is the mobile phase (it moves up the paper). Different substances in the mixture travel at different speeds, so they separate out into distinct spots or bands.

Worked example: Paper Chromatography of Food Colourings

Separate the different dyes in black ink using paper chromatography

1. Use pencil to draw a line 2cm from bottom of chromatography paper — Pencil won't dissolve in the solvent - pen ink would interfere with results
2. Place a small spot of black ink on the baseline — Keep the spot small and concentrated for better separation
3. Place paper in beaker with 1cm depth of water (below the baseline) — Solvent level must be below the ink spot or it will dissolve away
4. Wait for solvent to travel up the paper, carrying different dyes at different rates — Different dyes have different affinities for paper vs water, so separate

Answer: Different coloured bands showing the individual dyes that make up black ink

7. Choosing the Right Method

The key to separation is identifying which physical property differs between your substances. Each technique exploits a different property, so you need to match the method to the mixture.

Particle size differences? Use filtration. Solubility differences? Try crystallisation. Boiling point differences? Go for distillation. Need to identify unknown substances? Chromatography is your friend.

These separation techniques are everywhere in real life - from water treatment plants using filtration and chlorination, to oil refineries using fractional distillation to separate crude oil into petrol, diesel and other fractions. Understanding the physical principles helps you tackle any separation problem.