AQA GCSE Chemistry Foundation (8462), Foundation tier · Atomic structure and the periodic table › Atoms, isotopes and electronic structure
Practise Mixtures. 9 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What a mixture is and the physical processes used to separate one: filtration, crystallisation, simple distillation, fractional distillation and chromatography. Questions ask you to choose a method for a given mixture, justify it and describe how to carry it out.
Key facts
Mixture: two or more substances not chemically combined; each keeps its own properties
Physical separation: no chemical reaction, no new substances
Filtration: insoluble solid from a liquid (residue and filtrate)
Crystallisation: soluble solid from a solution
Simple distillation: solvent from a solution
Fractional distillation: liquids with different boiling points
Chromatography: separates dissolved substances, e.g. dyes
Condenser: cools the vapour so it condenses back to a liquid
Notes
What a mixture is
A mixture is two or more elements or compounds that are not chemically combined together.
Each substance in a mixture keeps its own chemical properties. For example, iron filings mixed with sand are still attracted to a magnet.
The amounts of each substance in a mixture can vary, unlike the fixed proportions in a compound. Air and sea water are mixtures.
Mixtures are separated by physical processes. These do not involve chemical reactions, so no new substances are made.
Separating solids from liquids
diagram
Filtration separates an insoluble solid from a liquid. The solid (the residue) stays on the filter paper; the liquid (the filtrate) passes through.
Filtration: the insoluble solid stays on the filter paper.
Crystallisation gets a soluble solid out of a solution. Heat the solution to evaporate some of the water, then leave it to cool so crystals form. Filter off the crystals and dry them.
Crystallisation gives better crystals than evaporating to dryness, and avoids breaking down solids that decompose when heated strongly.
Separating liquids
diagram
Simple distillation separates a solvent from a solution, e.g. pure water from salt water. The water boils, and its vapour is cooled in a condenser, where it condenses back to a liquid and is collected. The salt stays behind.
Simple distillation: the water boils, condenses in the condenser and is collected. The salt stays in the flask.
Fractional distillation separates liquids that mix, but have different boiling points, e.g. ethanol and water, or the fractions in crude oil.
A fractionating column is hottest at the bottom and coolest at the top. The liquid with the lowest boiling point reaches the top and is collected first; the thermometer reading rises when the next liquid starts to distil over.
Fractional distillation: the liquid with the lowest boiling point reaches the top first.
Chromatography
diagram
Chromatography separates substances dissolved in a solvent, e.g. the dyes in an ink.
The substances move different distances up the paper because they are attracted differently to the paper and to the solvent. (Rf values are covered in Chemical analysis.)
The start line is drawn in pencil, above the level of the solvent.
Choosing a method
Insoluble solid and a liquid → filtration.
Dissolved solid, and you want the solid → crystallisation.
Dissolved solid, and you want the solvent → simple distillation.
Liquids that mix, with different boiling points → fractional distillation.
Several dissolved substances, e.g. coloured dyes → chromatography.
How to answer each type of question
Suggest a separation method and give a reason
1 to 2 marksGrade 4
Decide what is in the mixture: insoluble solid, dissolved solid or liquids.
Decide what you want to keep.
Name the method and give the property it relies on: solubility or boiling point.
Example. A student has a mixture of sand and copper sulfate solution. Name the method the student should use to remove the sand. Give a reason for your answer.
Show the model answerHide the model answer
Filtration (1). Sand is insoluble (in water), so it stays on the filter paper while the solution passes through (1).
Explain how distillation separates a mixture
2 to 3 marksGrade 5
Say what boils (evaporates) and why the other substance does not.
Say that the vapour is cooled in the condenser.
Say that it condenses to a liquid, which is collected.
Example. Explain how simple distillation produces pure water from sea water.
Show the model answerHide the model answer
The water boils, but the dissolved salts do not because they have much higher boiling points (1). The water vapour passes into the condenser, where it is cooled (1). It condenses back to liquid water, which is collected (1).
Don’t lose marks
Saying filtration separates salt from salt water. Dissolved substances pass straight through filter paper.
Mixing up soluble and insoluble.
Choosing evaporation when the question wants the solvent. Evaporating loses the water; distillation collects it.
Saying that separating a mixture makes new substances or involves a chemical reaction.
Describing crystallisation as heating until all the water has gone.
More tips
Memory tricks
Ask two questions every time: does it dissolve, and what do I want to keep?
Distil = boil, then condense. Fractional = more than one liquid to collect.
The residue 'resides' in the filter paper; the filtrate 'filters through'.
Condenser water goes in at the bottom and out at the top, so the condenser stays full of cold water.
Exam technique
For 'describe how' questions, write the steps in order and name the equipment: beaker, filter funnel and paper, evaporating basin, condenser.
Always justify a choice with a property: solubility for filtration and crystallisation, boiling point for distillation.
Use the specification's names exactly: filtration, crystallisation, simple distillation, fractional distillation, chromatography.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define a mixtureTwo or more elements or compounds that are not chemically combined together.
Grade 4
Match each separation method to its useFiltration, crystallisation, simple distillation, fractional distillation or chromatography, depending on what is in the mixture.
Grade 4
Describe filtration and crystallisationFilter off an insoluble solid; evaporate some water from a solution, then cool it so crystals of the dissolved solid form.
Grade 5
Describe simple distillationThe solvent boils off, is cooled and condensed in a condenser, and is collected, leaving the dissolved solid behind.
Grade 5
Describe fractional distillationLiquids with different boiling points are separated using a fractionating column; the lowest boiling point liquid is collected first.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
This question is about mixtures and how they are separated. What is a mixture?
Two or more elements or compounds that are not chemically combined.
A student heated blue copper(II) sulfate solution in a flask. The flask was connected to a water-cooled condenser. A thermometer was placed at the top of the flask, level with the side arm that led to the condenser. A liquid was collected in a beaker at the end of the condenser. Name this separation technique.
simple distillation
Suggest a mixture from which a solid could be obtained by heating the mixture in an evaporating basin.
e.g. salt solution (salt dissolved in water)
Name the method that should be used to separate ethanol from mixture 1.
fractional distillation
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
This question is about mixtures and how they are separated.
(a) Which method would separate sand from a mixture of sand and water? Tick (✓) one box.[1]
Chromatography
Crystallisation
Filtration
Fractional distillation
(b) Which method would separate the different coloured dyes in a sample of ink? Tick (✓) one box.[1]
Chromatography
Crystallisation
Filtration
Fractional distillation
(c) What is a mixture?[2]
(d) Separating a mixture does not make any new substances. What type of process is used to separate a mixture?[1]
Show the answer and mark scheme
(a)Answer: Filtration
(b)Answer: Chromatography
(c)Answer: Two or more elements or compounds that are not chemically combined.
two or more elements or compounds / substances
not chemically combined (together)
(d)Answer: a physical process
physical (process)
Question 2Medium6 marks
A student heated blue copper(II) sulfate solution in a flask. The flask was connected to a water-cooled condenser. A thermometer was placed at the top of the flask, level with the side arm that led to the condenser. A liquid was collected in a beaker at the end of the condenser.
(a) Name this separation technique.[1]
(b) Describe what happens to the water in the flask and in the condenser.[2]
(c) The thermometer read 100 °C while the liquid was being collected. What does this show about the liquid collected?[1]
(d) What colour is the liquid collected in the beaker? Give a reason for your answer.[2]
Show the answer and mark scheme
(a)Answer: simple distillation
(simple) distillation
(b)Answer: The water boils/evaporates in the flask; the water vapour is cooled and condenses in the condenser.
water evaporates / boils (in the flask)
(water vapour / steam) is cooled and condenses (in the condenser)
(c)Answer: It is (pure) water, because water boils at 100 °C.
it is (pure) water / 100 °C is the boiling point of water
(d)Answer: Colourless; the copper(II) sulfate does not evaporate, so it stays in the flask.
colourless
copper(II) sulfate does not evaporate / boil (at 100 °C) so it stays in the flask