AQA GCSE Chemistry (8462), Higher tier · 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.
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