AQA GCSE Combined Science (8464), Higher tier · Chemistry › Chemical analysis › Purity, formulations and chromatography
Practise Pure substances. 12 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 'pure' means in chemistry, and how melting point and boiling point data tell a pure substance from a mixture. Expect short definitions and questions where you interpret melting or boiling temperatures from tables or heating curves.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define a pure substance in chemistryA single element or a single compound, not mixed with any other substance.
4
Contrast everyday and chemical meanings of pureIn everyday language 'pure' means nothing has been added (e.g. pure milk), but the product can still be a mixture.
4
Use melting points to spot pure samplesA pure substance melts at a specific temperature; a mixture melts over a range of temperatures.
5
Describe how impurities change melting and boilingImpurities lower the melting point and widen the melting range; dissolved impurities raise the boiling point.
5
Interpret heating curves to judge purityA flat section at a fixed temperature shows a pure substance changing state; a sloping section shows a mixture.
7
Compare data to rank samples by purityThe purer sample melts closer to the pure value and over a narrower range of temperatures.
Notes
What 'pure' means in chemistry
In chemistry, a pure substance is a single element or a single compound, not mixed with any other substance.
Examples: oxygen, copper, distilled water, sodium chloride. Air, sea water, steel and milk are mixtures, so they are not pure.
In everyday language, 'pure' means that nothing has been added, so the substance is in its natural state, e.g. 'pure orange juice' or 'pure milk'. These products are still mixtures of many substances.
Using melting and boiling points
A pure element or compound melts and boils at a specific temperature. For example, pure water melts at 0 °C and boils at 100 °C.
A mixture (an impure substance) melts and boils over a range of temperatures.
Impurities lower the melting point and widen the melting range. The more impurity, the lower and wider the range.
Dissolved impurities raise the boiling point. Salty water boils above 100 °C, and its temperature keeps rising as it boils.
To test purity, measure the melting point or boiling point and compare it with the value for the pure substance.
Reading heating curves
On a heating curve (temperature against time) for a pure substance, the line is flat while it melts or boils: the temperature stays constant.
For a mixture, the line slopes during melting or boiling, because the change of state happens over a range of temperatures.
A sharp melting point shows a sample is pure, but it does not prove which substance it is: different compounds can have the same melting point. grade 7+
Cheatsheet
Pure substance = a single element or compound, not mixed with any other substance
Everyday 'pure' = nothing added, natural state (it can still be a mixture)
Pure substance: melts and boils at a specific temperature
Mixture: melts and boils over a range of temperatures
Impurities lower the melting point and widen the melting range
Dissolved impurities raise the boiling point
Pure water: melts at 0 °C, boils at 100 °C
Heating curve: flat section = pure substance changing state
How to answer each type of question
Define pure, or explain the everyday and chemical meanings
1 to 3 marks3
Use the chemical definition: a single element or compound, not mixed with any other substance.
For the everyday meaning, say that nothing has been added / it is in its natural state.
Explain why the product is not chemically pure: it contains more than one substance, so it is a mixture.
Example. A bottle of water is labelled 'Pure spring water'. (a) What does the word 'pure' mean on this label? (b) Explain why the spring water is not pure in the chemical sense.
Show the model answer
(a) Nothing has been added to it / it is in its natural state (1) (b) A pure substance is a single element or compound, not mixed with anything else (1). Spring water contains dissolved substances as well as water, so it is a mixture (1).
Use melting point data to pick out the pure sample
2 marks4
Look for the sample that melts at a single, specific temperature.
Check that this temperature matches the value given for the pure substance.
Give the reason using the data: 'melts at a specific temperature', not 'has a melting point'.
Example. Pure compound Z melts at 114 °C. A student measured the melting temperatures of three samples labelled Z. Sample A: melted from 105 °C to 111 °C Sample B: melted at 114 °C Sample C: melted from 110 °C to 113 °C Which sample is pure Z? Give a reason for your answer.
Show the model answer
Sample B (1). It melts at a specific temperature, 114 °C, which is the melting point of pure Z (1).
Interpret a heating curve
2 marks5
Find the part of each curve where the substance is changing state.
Flat (constant temperature) means a pure substance; still rising means a mixture.
Name the substance and give the reason from the graph.
Example. Two colourless liquids, P and Q, were heated until they boiled. The temperature of P rose to 82 °C and then stayed at 82 °C for 5 minutes while P boiled. The temperature of Q rose to 85 °C and then kept rising slowly, to 91 °C, while Q boiled. Which liquid is a mixture? Explain your answer.
Show the model answer
Q (1). Its temperature keeps rising while it boils, so it boils over a range of temperatures instead of at a fixed temperature (1).
Explain what data show about the purity of a sample
2 to 3 marks6
Compare with the pure value: is the melting point lower, or the boiling point higher?
Say whether it changes state at a fixed temperature or over a range.
State your conclusion: pure, or impure (a mixture).
Example. Pure water boils at 100 °C. A student heated a sample of river water. It started to boil at 101 °C and the temperature rose to 103 °C as it boiled. Explain what these results show about the river water.
Show the model answer
The river water is not pure / is a mixture (1). It boils at a higher temperature than pure water (1) and over a range of temperatures, not at one fixed temperature (1).
Shortcuts and memory tricks
Count the substances: one substance means pure; two or more means a mixture.
Sharp means pure; a range means impure.
Impurities push the melting point DOWN and the boiling point UP: 'melt down, boil up'.
Quick check: the purer sample has the narrower melting range, closer to the pure value.
Where marks are lost
Giving the everyday meaning ('nothing added', 'natural') when the question asks for the chemical meaning.
Saying impurities raise the melting point. They lower it and make it melt over a range.
Writing 'it has a melting point' as the reason. Every solid has one: say it melts at a specific (fixed) temperature.
Picking a sample as pure just because it melts at a high temperature. Purity is shown by melting at one specific temperature that matches the pure substance.
Calling an alloy, air or a food 'pure'. They are all mixtures.
Exam technique
Learn the definition word for word: 'a single element or compound, not mixed with any other substance'.
Quote the data, e.g. 'it melts from 105 °C to 111 °C, which is a range, and is below 114 °C'.
In 'explain' questions give both ideas: lower (or higher) than the pure value AND a range instead of a fixed temperature.
On a heating curve, find the flat section and read its temperature carefully from the axis.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
In chemistry, the word 'pure' has a special meaning.
(a) Which substance is pure in the chemical sense? Tick (✓) one box.[1]
Air
Copper
Fresh orange juice with nothing added
Steel
(b) What is meant by a pure substance in chemistry?[1]
(c) A carton of milk is labelled 'pure milk'. Explain why the milk is described as pure in everyday language but is not pure in the chemical sense.[2]
Show the answer and mark scheme
(a)Answer: Copper
(b)
a single element or (a single) compound (not mixed with any other substance)
(c)
(in everyday language pure means) nothing has been added / it is in its natural state
(in chemistry) milk is a mixture / milk contains different substances (e.g. water, fats, proteins)
Question 2Medium3 marks
(a) Describe how a student could measure the melting point of a small sample of a solid in the laboratory.[2]
(b) Suggest why only a small amount of solid is used.[1]
Show the answer and mark scheme
(a)
put a small amount of the solid in a melting point tube (a thin capillary tube) attached to a thermometer, or in an electric melting point apparatus
heat slowly and record the temperature at which the solid starts to melt and the temperature at which it has completely melted
(b)
a small sample heats through evenly and quickly, so the temperatures at which it starts and finishes melting can be measured precisely
Question 3Hard5 marks
A pharmaceutical company makes a drug, Z. A pure sample of Z melts at 182 °C. The company tests each batch of Z by measuring its melting point before selling it.
(a) One batch of Z melted between 175 °C and 181 °C. Evaluate what this test result shows, and suggest one further test the company could do to find out more about the batch.[3]
(b) Explain why a sharp melting point of 182 °C would not prove with certainty that a sample is pure Z.[2]
Show the answer and mark scheme
(a)
the batch is impure: it melts below 182 °C and over a range, rather than sharply at 182 °C
the melting point test does not show what the impurity is or how much of it is present
paper chromatography could be used, comparing the batch with a pure reference sample of Z, to see how many substances are present
(b)
a very small amount of impurity might lower the melting point by an amount too small to detect as a range
a different pure substance could, by chance, have the same (or a very similar) melting point as Z