AQA Chemistry (8462) 4.1.1.2 · Classify
Mixtures and separation techniques
Stir sugar into water and it seems to vanish. How do you get each substance back out of a mixture, and how do you choose the right method?
Triple Science · Higher tier
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A spoonful of sugar in hot water.
The crystals vanish. The water looks exactly as clear as before, but it now tastes sweet.
If you had to guess, is it still sugar, or has it turned into something new?
Sugar water is a mixture: two or more elements or compounds that are not chemically combined together. Each substance keeps its own chemical properties, so a mixture can be separated by physical processes, which make no new substances. You need to know five: filtration, crystallisation, simple distillation, fractional distillation and chromatography.
Choosing one comes down to two questions. What is different about the substances: does one dissolve while the other does not, or do they boil at different temperatures? And which substance do you want to keep?
The separation desk
Blue ink: dyes dissolved in water
0 of 6 decided
You want pure water. Which method?
Think again
“Use a fine enough filter paper and it will catch the salt.”
Predict
ExplainSalt water is poured through the finest filter paper there is. What comes through?
Build the method
Pure, dry copper sulfate crystals from a mixture of copper sulfate and sand
Put the steps in order.
4 links belong. 2 do not.
The links
Your chain · 0 of 4
In the lab, the fractional distillation column is packed with glass beads: the vapour condenses and boils again many times on its way up. The fractions are collected one after another as the thermometer reading rises, changing the beaker each time.
Practical skills · setting up apparatus
IdentifyA student set up this simple distillation to collect pure water from ink. Which part must be changed?
Command words in this lesson
- Give
- The answer only, no reasons: for a mixture, “not chemically combined” is the phrase that scores.
- Suggest
- Use the information given, such as boiling points, to choose a method you may not have seen used on that mixture.
- Explain
- Name the property that differs, then say what it lets you collect.
- Plan
- A method that would work: the steps in order, the apparatus, and what you end up with.
Key fact
The substances in a mixture are not chemically combined, so each keeps its own properties. Separation uses a difference in a physical property, such as solubility or boiling point, and no new substance is made.
Key note · AQA 4.1.1.2 (8462)
Mixtures and separation techniques
- Mixture: two or more elements or compounds not chemically combined; separated by physical processes that make no new substances.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
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Not sure which separation method to choose?
The apparatus is drawn as simple outlines, without clamps and stands, and each separation runs in seconds and comes out clean. Real filtration, crystallisation and distillation take minutes to hours, and fractional distillation of ethanol and water can never give pure ethanol: some water always comes over with it.