AQA GCSE Chemistry (8462), Higher tier · Chemical analysis › Identification of ions
Practise Instrumental methods. 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.
Required practical:Identifying ions (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
An instrument measures the concentration of metal ions in water samples. Its display shows readings such as 0.037 mg/dm3 and 0.041 mg/dm3. State the resolution of this instrument.
0.001 mg/dm3
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
Elements and compounds can be detected and identified using instrumental methods.
(a) Which two statements are advantages of instrumental methods compared with chemical tests? Tick (✓) two boxes.[2]
They are accurate
They are cheap to buy
They are rapid
They need large samples
They need no training to use
(b) Instrumental methods are also sensitive. What does 'sensitive' mean?[1]
Show the answer and mark scheme
(a)Answer: They are accurate; They are rapid
(b)
can detect very small amounts / very low concentrations of a substance
Question 2Medium5 marks
A water company tests 2000 samples of drinking water each day for harmful metal ions. The company uses instrumental methods rather than chemical tests.
(a) Suggest two reasons why the company uses instrumental methods.[2]
(b) Suggest one disadvantage of instrumental methods.[1]
(c) Lead ions are harmful even at very low concentrations. Explain why a precipitation test would not be suitable for checking that drinking water is safe.[2]
Show the answer and mark scheme
(a)
rapid / quick so many samples can be tested each day
sensitive / can detect very low concentrations of metal ions
accurate (results are reliable)
can be automated / need fewer staff
(b)
the equipment is expensive / needs trained operators / must be calibrated using reference samples
(c)
the concentration of lead ions may be too low to form a visible precipitate / the test is not sensitive enough
so a harmful concentration could go undetected / the test does not measure the concentration
Question 3Hard3 marks
Before using an instrument to measure the concentration of a metal ion in unknown samples, a scientist first measures the instrument's response for several solutions of known concentration.
(a) Explain why this step is necessary.[2]
(b) Suggest what the scientist should do if an unknown sample gives a response higher than any of the known solutions used for calibration.[1]
Show the answer and mark scheme
(a)
it produces a calibration curve / set of reference readings linking the instrument's response to concentration
so that the response measured for an unknown sample can be converted into an accurate concentration
(b)
dilute the unknown sample by a known factor and re-test it, then multiply the result by the dilution factor