How do we tell pure substances from mixtures and separate the components?
Mixtures and separation: pure substances and mixtures, the separation techniques (filtration, crystallisation, simple and fractional distillation), and producing potable water.
A focused answer to Edexcel GCSE Chemistry topic 2, covering the difference between pure substances and mixtures, how purity affects melting and boiling points, filtration, crystallisation, simple and fractional distillation, choosing the right technique, and producing potable water.
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What this dot point is asking
Edexcel wants you to distinguish a pure substance from a mixture, explain how purity affects melting and boiling points, describe filtration, crystallisation and simple and fractional distillation, choose the right technique for a given mixture, and outline how potable water is produced. The separation methods are tested with apparatus and ordered method steps.
Pure substances and mixtures
Purity affects the melting and boiling behaviour, which is how a chemist tests purity:
- A pure substance melts and boils at a single, sharp temperature.
- A mixture melts and boils over a range of temperatures, and the presence of impurity usually lowers the melting point and raises the boiling point.
In everyday language "pure" can mean natural or unadulterated (for example pure orange juice), but in chemistry it means a single substance only.
Filtration
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel: the solid is trapped as the residue and the liquid passes through as the filtrate. It is used to separate, for example, sand from salt solution.
Crystallisation
Crystallisation recovers a soluble solid from its solution. The solution is heated to evaporate some of the water until it is saturated, then left to cool slowly so that crystals form. Gentle evaporation avoids decomposing the solid. It is used to obtain salt crystals from salt solution.
Distillation
Simple distillation separates a solvent from a solution (for example pure water from salt solution). The solution is heated, the solvent evaporates, and the vapour is cooled in a condenser and collected as the distillate, leaving the dissolved solid behind.
Fractional distillation separates two or more miscible liquids with different boiling points (for example ethanol from water, or the fractions of crude oil). A fractionating column above the flask gives repeated evaporation and condensation, so the liquid with the lower boiling point reaches the top and is collected first.
Producing potable water
In the UK, potable water is produced by:
- Choosing a suitable source (such as a reservoir of fresh water).
- Passing it through filter beds to remove insoluble solids.
- Sterilising it to kill microbes, using chlorine, ozone or ultraviolet light.
Where fresh water is scarce, sea water can be desalinated by distillation or reverse osmosis, but this needs a lot of energy.
Try this
Q1. State one way a pure substance differs from a mixture in its melting behaviour. [1 mark]
- Cue. A pure substance melts at a single sharp temperature; a mixture melts over a range.
Q2. Which technique separates an insoluble solid from a liquid? [1 mark]
- Cue. Filtration.
Q3. Describe the three main steps used to make river water potable. [3 marks]
- Cue. Choose a suitable source; filter to remove insoluble solids; sterilise with chlorine, ozone or ultraviolet light to kill microbes.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20184 marksA student has a mixture of sand and salt. Describe how the student could obtain pure, dry samples of both the sand and the salt from the mixture.Show worked answer →
A 4-mark separation-method question requiring an ordered method.
Add water and stir to dissolve the salt, leaving the insoluble sand (1 mark). Filter the mixture: the sand is the residue trapped in the filter paper and the salt solution is the filtrate (1 mark). Wash and dry the sand to obtain it pure (1 mark). Evaporate or crystallise the filtrate by heating to remove the water, leaving pure salt crystals (1 mark).
Markers reward the correct order (dissolve, filter, then crystallise) and naming the residue and filtrate.
Edexcel 20213 marksExplain why fractional distillation, rather than simple distillation, is used to separate a mixture of two liquids such as ethanol and water that have boiling points close together.Show worked answer →
A 3-mark technique-justification question.
Fractional distillation uses a fractionating column, which gives repeated evaporation and condensation up the column (1 mark). This separates liquids whose boiling points are close together, because the vapour becomes richer in the lower-boiling liquid as it rises (1 mark). Simple distillation cannot separate them cleanly because both liquids would evaporate together over a similar temperature range, giving an impure distillate (1 mark).
Markers reward the role of the fractionating column and the idea of repeated evaporation and condensation.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)