Separating Mixtures: Filtration and Distillation KS3

A mixture contains two or more substances that are not chemically combined and can be separated by physical methods. The best technique depends on the physical properties of the substances in the mixture. Filtration, distillation, evaporation, and chromatography are the four main separation methods at KS3.

What is a mixture?

A mixture is formed when two or more substances are combined but not chemically bonded together. The substances in a mixture keep their own properties and can be separated by physical means. Examples include:

Salt water (salt dissolved in water)

Sand and gravel

Ink (a mixture of different coloured dyes)

Air (a mixture of nitrogen, oxygen, argon, and other gases)

A mixture is different from a compound: in a compound, elements are chemically bonded together in fixed ratios and can only be separated by chemical reactions.

Filtration

Filtration separates an insoluble solid from a liquid. The mixture is poured through a filter paper inside a funnel. The liquid passes through the tiny pores in the filter paper — this liquid is called the filtrate. The solid particles are too large to pass through and are trapped — this is called the residue.

Worked example: separating sand from salt water

Suppose you have a mixture of sand, salt, and water. You want to recover the salt.

Step 1 — Filtration: Pour the mixture through filter paper in a funnel. The sand (insoluble) stays behind as the residue. The salt water (the filtrate) drips through.

Step 2 — Evaporation: Heat the salt water in an evaporating basin. The water evaporates (turns to steam) and is lost to the air. The salt crystals are left behind in the basin.

This two-step process — filtration followed by evaporation — is a classic KS3 practical. Note that evaporation is used when you want to recover the solute (the dissolved salt), not the solvent (water).

Simple distillation

Simple distillation separates a solvent from a solution (or separates two liquids with very different boiling points). It relies on the fact that different liquids boil at different temperatures.

Apparatus and method

The solution is heated in a round-bottomed flask. The solvent (with the lower boiling point) boils and turns to vapour.

The vapour passes through a condenser — a water-cooled tube — where it cools and condenses back into a liquid called the distillate.

The distillate is collected in a separate container. The dissolved solid (or higher-boiling-point liquid) remains in the flask.

Worked example: obtaining pure water from salt water

Salt dissolves in water to make salt water (a solution). To obtain pure water:

Heat the salt water. Water boils at 100°C; salt does not boil (its boiling point is around 1,413°C — far higher).

Water vapour travels up the flask and into the condenser.

The condenser cools the vapour back to liquid water (distillate), collected in a beaker.

Pure salt remains in the original flask.

This is how desalination plants produce drinking water from seawater — though industrial plants use more energy-efficient methods than lab distillation.

Fractional distillation

Fractional distillation separates a mixture of liquids whose boiling points are close together (for example, different fractions of crude oil, or ethanol from water).

A fractionating column is used: it maintains a temperature gradient, so liquids with lower boiling points rise further up the column and condense at the top, while liquids with higher boiling points condense lower down and are collected separately.

Fractional distillation is used industrially to:

Separate crude oil into petrol, diesel, kerosene, and lubricating oil fractions.

Produce industrial alcohol from fermented mixtures.

Separate liquid air into nitrogen (boiling point −196°C), oxygen (−183°C), and argon (−186°C).

Chromatography

Chromatography separates mixtures of soluble substances (typically dyes or inks) based on how quickly they travel through a stationary phase (usually paper or gel) carried by a moving solvent.

Paper chromatography: method

Draw a pencil baseline near the bottom of a strip of chromatography paper (pencil, not pen — pen ink would dissolve and move).

Place small spots of the mixtures to be tested on the baseline.

Place the paper in a solvent (e.g., water or ethanol) so that the solvent level is just below the spots.

Allow the solvent to travel up the paper. Different substances travel different distances because they have different solubility in the solvent and different affinity for the paper.

Mark the solvent front when it nearly reaches the top. Remove the paper and let it dry.

Interpreting a chromatogram

Each spot on the chromatogram represents one substance. If a spot from an unknown mixture lines up at exactly the same height as a spot from a known reference substance, they are likely the same compound.

The Rf value (retention factor) can be calculated to compare results:

Rf = distance travelled by substance ÷ distance travelled by solvent front

Rf values are always between 0 and 1 and are characteristic for a given substance in a given solvent.

Worked example: testing food colourings

A food technologist wants to check whether an orange food dye contains only permitted colourants. They spot three commercial food dyes (A, B, C) and the unknown orange dye on chromatography paper and run the chromatogram.

Results: the unknown dye produces two spots. One spot matches dye A (Rf = 0.45) and one matches dye B (Rf = 0.71). Dye C (Rf = 0.30) does not appear. Conclusion: the orange dye is a mixture of dye A and dye B.

Which separation technique should you choose?

Situation | Technique
Insoluble solid + liquid | Filtration
Soluble solid from solution (keep solid) | Evaporation
Pure solvent from solution | Simple distillation
Mixed liquids with similar boiling points | Fractional distillation
Mixed soluble substances (dyes, inks) | Chromatography

According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should be taught to use appropriate separation techniques based on the properties of the substances involved. BBC Bitesize KS3 Chemistry describes filtration, evaporation, distillation, and chromatography as the four key methods, with worked examples for each.

Frequently asked questions

What is the difference between filtration and distillation?

Filtration separates an insoluble solid from a liquid using a filter paper — the solid is trapped and the liquid passes through. Distillation separates a solvent from a solution by boiling the solvent off and condensing it elsewhere. You use filtration when the solid does not dissolve; you use distillation when the solid is dissolved.

What is the residue and what is the filtrate in filtration?

The residue is the solid left behind on the filter paper — the part that could not pass through. The filtrate is the liquid that passes through the filter paper and is collected below.

What is the Rf value in chromatography?

The Rf value (retention factor) is the ratio of the distance a substance travels to the distance the solvent travels, measured from the baseline. Rf = distance moved by substance ÷ distance moved by solvent front. It is always between 0 and 1. Rf values help identify unknown substances by comparison with known references.

Why is pencil used to draw the baseline in paper chromatography?

Pencil (graphite) is insoluble in the solvents used in chromatography, so the pencil mark stays in place as the solvent moves up the paper. If pen ink were used, the ink would dissolve and travel up the paper, mixing with the test substances and confusing the results.

How does fractional distillation differ from simple distillation?

Simple distillation works when there is a large difference in boiling points between the solvent and the dissolved substance. Fractional distillation uses a fractionating column to separate liquids with similar boiling points. The column maintains a temperature gradient, allowing substances to separate at different heights based on their boiling points.

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