AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical analysis › Identification of ions
Practise Instrumental methods. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Instrumental methods use machines to detect and identify elements and compounds. This is in GCSE Chemistry only (not Combined Science). You need to state their advantages (accurate, sensitive and rapid) and compare them with the chemical tests in the specification.
Key facts
Instrumental method = uses an instrument to detect and identify substances
Advantages: accurate, sensitive, rapid
Accurate = close to the true value
Sensitive = detects very small amounts
Rapid = gives results quickly
Can analyse mixtures and measure amounts
Chemical tests: cheap and simple, but less sensitive and unclear with mixtures
Instruments: expensive, need trained staff
Notes
What they are
Instrumental methods use instruments (machines, often computer-controlled) to detect and identify elements and compounds.
They are used, for example, in medicine, forensic science, environmental monitoring and checking the quality of products in industry.
The one you study in detail is flame emission spectroscopy.
Advantages
Accurate: the results are close to the true value.
Sensitive: they can detect and measure very small amounts of a substance, so only a tiny sample is needed.
Rapid: they give results quickly, and many samples can be analysed automatically.
Many can analyse mixtures and measure how much of each substance is present.
Compared with chemical tests
Chemical tests (gas tests, flame tests, precipitate tests) are cheap, use simple equipment and can be done almost anywhere.
But chemical tests need more of the sample, can give unclear results with mixtures (e.g. masked flame colours), rely on judging colours by eye, and usually show only which ion is present, not how much.
Instruments are expensive to buy and run, need trained operators, and their results must be compared with reference data for known substances.
How to answer each type of question
State advantages of instrumental methods
1 to 3 marksGrade 4
Use the three spec words: accurate, sensitive, rapid.
If the question says 'compared with chemical tests', make each point a comparison: 'more sensitive', 'faster'.
If there are more marks, explain each point, e.g. 'sensitive, so it detects very small amounts'.
Example. Give two advantages of using instrumental methods rather than chemical tests to identify the metal ions in a sample of river water.
Show the model answerHide the model answer
Any two, 1 mark each: more sensitive / detects very small amounts (1); more accurate (1); faster / more rapid (1); can identify the ions in a mixture (1); can measure how much of each ion is present (1). Maximum 2 marks.
Don’t lose marks
Writing vague answers like 'easier' or 'better'. Use the spec words accurate, sensitive, rapid.
Confusing sensitive with accurate: sensitive means it detects small amounts; accurate means close to the true value.
Giving only advantages in an 'evaluate' question. Include disadvantages (cost, trained staff) and a conclusion.
Saying chemical tests are useless. They work well for single ions in reasonable amounts, cheaply and quickly.
More tips
Memory tricks
The three advantages, A S R: Accurate, Sensitive, Rapid ('Always Super Rapid').
Sensitive = small: tiny samples, tiny amounts.
For evaluate questions, score both methods on a quick grid: cost, speed, sample size, mixtures, amounts.
Exam technique
In comparisons, use comparative words: 'more sensitive', 'faster', 'cheaper'.
Tie each point to the context, e.g. 'sensitive, so the tiny speck of paint can be analysed'.
Evaluate = pros and cons of each method, then a justified judgement at the end.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what instrumental methods areMethods that use instruments (machines) to detect and identify elements and compounds.
Grade 4
State the three advantages of instrumental methodsThey are accurate, sensitive and rapid.
Grade 5
Explain accurate, sensitive and rapidAccurate: close to the true value. Sensitive: detects very small amounts. Rapid: gives results quickly.
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