Edexcel IGCSE Chemistry · Spec 1.10
Separating Mixtures
Separating mixtures by filtration, crystallisation, evaporation and distillation.
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Separating Mixtures
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Explained
Choosing the right separation, and doing it properly
A mixture can be separated physically, because its components are not chemically joined. Which method you use depends entirely on what is in the mixture, so the first question is always what kind of mixture you have.
Filtration
Use it to separate an insoluble solid from a liquid, such as sand from water.
The mixture is poured through filter paper in a funnel. The solid is trapped and is called the residue; the liquid passes through and is called the filtrate. Those two words are worth having ready, because questions ask which is which.
Crystallisation
Use it to get a soluble solid back out of its solution, such as salt from salt water.
Heat the solution to evaporate some of the water, until crystals just start to form at the edge. Then stop heating and leave the solution to cool. As it cools, less solute can stay dissolved, so crystals form. Pour off the remaining liquid and dry the crystals in a warm place.
Stopping the heating at the right moment matters. Boiling the solution dry gives you a powder contaminated with any impurities, rather than clean crystals.
Simple distillation
Use it to separate a liquid from a solution, when you want the liquid, such as pure water from seawater.
Heat the solution. The water boils, the vapour passes into a condenser where it cools back to a liquid, and it is collected. The dissolved solid stays behind in the flask because it has a far higher boiling point.
Fractional distillation
Use it to separate two or more liquids that mix, when their boiling points are different, such as ethanol from water.
A fractionating column above the flask gives repeated evaporation and condensation on the way up. The liquid with the lower boiling point reaches the top first and is collected; the other flows back down. Watch the thermometer at the top and change the collecting flask when the temperature jumps.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks which process separates a mixture of liquids and credits fractional distillation, allowing fractionating or fractionation, then adds: ignore distillation alone.
The word fractional is doing the work. Simple distillation separates a liquid from a dissolved solid; fractional distillation separates liquids from each other. Leaving the adjective off names the wrong process.
On crystallisation, a mark scheme sets out five steps from six possible: filter off the excess solid, heat, until crystals first start to form, allow to cool and crystallise, pour off the excess liquid, and leave in a warm place to dry. It allows decanting for pouring off and drying in a warm oven or a desiccator, and it ignores washing the crystals.
Then comes the ruling that catches people: if the sample is heated to dryness, only the first two marks are awarded. Three of the five marks are lost by one wrong instruction, because heating to dryness is not crystallisation at all.
Chromatography
Use it to separate substances dissolved in the same solvent, such as the dyes in an ink.
Spot the mixture on a pencil baseline near the bottom of the paper, and stand it in solvent with the baseline above the solvent level. The solvent rises, carrying the substances with it at different rates according to how soluble they are and how strongly they cling to the paper.
The baseline is drawn in pencil because pencil is insoluble and will not travel up with the solvent. The baseline sits above the solvent because otherwise the spots would dissolve into it and be washed away rather than carried up.
The Rf value is the distance moved by the spot divided by the distance moved by the solvent front. It has no unit and is always less than one, so an answer above one means the two measurements have been divided the wrong way round.
Spec 1.10
What you need to know
- Choose the right method for a given mixture
- Describe filtration and crystallisation
- Describe simple and fractional distillation
Active recall
Quick check
Answer each question before opening the answer.
Which method separates an insoluble solid from a liquid, and a dissolved solid from its solution?
Filtration for the insoluble solid; crystallisation (or evaporation) for the dissolved solid.
How would you separate two miscible liquids with different boiling points?
Fractional distillation.
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