AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical analysis › Identification of ions
Practise Flame emission spectroscopy. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Flame emission spectroscopy is an instrumental method that identifies the metal ions in a solution from their line spectra and measures their concentrations. This is in GCSE Chemistry only (not Combined Science). You interpret spectra or data by comparing them with reference spectra.
Key facts
Sample solution in flame → light → spectroscope → line spectrum
Each metal ion has its own pattern of lines
A metal ion is present only if ALL its reference lines appear
Mixture: the spectrum shows the lines of every metal ion present
More intense lines = higher concentration
Better than flame tests: no masking, gives concentrations, more sensitive
Notes
How it works
diagram
Flame emission spectroscopy is an instrumental method used to analyse metal ions in solutions.
The sample solution is put into a flame, and the metal ions give out light.
The light given out is passed through a spectroscope, which separates it into different wavelengths.
The output is a line spectrum: a pattern of lines at particular wavelengths.
Each metal ion gives its own pattern of lines.
Identifying metal ions
diagram
Each metal ion gives its own pattern of lines, like a fingerprint.
Compare the sample's spectrum with reference spectra of known metal ions. A metal ion is present only if all of its lines appear in the sample's spectrum.
Sample contains A and B: all their lines appear. C is absent: its 450 nm line is missing.
A mixture gives a spectrum containing the lines of every metal ion present, so each ion can still be identified. With flame tests, one colour can mask another.
Measuring concentration
The intensity (brightness) of the lines depends on the concentration of the metal ion: the higher the concentration, the more intense the lines.
Compared with flame tests, it is more sensitive and accurate, works with mixtures and gives concentrations, but the equipment is expensive.
How to answer each type of question
Describe how flame emission spectroscopy is used
2 to 3 marksGrade 4
The sample solution is put into a flame.
The light given out is passed through a spectroscope to give a line spectrum.
The spectrum is compared with reference spectra of known metal ions.
Example. Describe how flame emission spectroscopy is used to identify the metal ion in a solution.
Show the model answerHide the model answer
The solution is put into a flame (1). The light given out is passed through a spectroscope to give a line spectrum (1). The spectrum is compared with reference spectra of known metal ions to find the ion whose lines match (1).
Don’t lose marks
Saying a metal ion is present when only some of its lines match.
Forgetting that a mixture's spectrum contains the lines of every ion present.
Describing a flame test (a colour judged by eye) instead of a line spectrum from a spectroscope.
Reading a calibration graph the wrong way: start from the intensity, then read off the concentration.
More tips
Memory tricks
A line spectrum is a fingerprint: a metal ion is only there if every one of its lines is there.
Tick off the lines: cross out each sample line as you match it. Any line left over means another ion is present.
Brighter lines = more ions = higher concentration.
Exam technique
Quote the wavelengths when you explain a match, e.g. 'lines at 470, 610 and 650 nm all appear'.
For 'describe', use three steps: flame → spectroscope → line spectrum compared with reference spectra.
If asked for an advantage over flame tests, mention mixtures (no masking) and measuring concentration.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Describe how flame emission spectroscopy worksThe sample is put into a flame and the light given out passes through a spectroscope, giving a line spectrum.
Grade 5
Identify a metal ion from reference spectraEach metal ion gives its own pattern of lines; the ion is present if all its lines appear.
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.
Name the piece of equipment used to produce and record the line spectrum of a sample in flame emission spectroscopy.
a spectroscope
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
Flame emission spectroscopy is an instrumental method used to analyse metal ions in solutions. The sample is put into a flame and the light given out is passed through a spectroscope.
(a) What is the output from the spectroscope called? Tick (✓) one box.[1]
A chromatogram
A line spectrum
A precipitate
A reaction profile
(b) Give two things that the output can be used to find out about the metal ions in the solution.[2]
Show the answer and mark scheme
(a)Answer: A line spectrum
(b)
which metal ions are present / the identity of the metal ions
the concentration of the metal ions
Question 2Medium2 marks
Explain the similarity and the key difference between a flame test and flame emission spectroscopy.[2]
Show the answer and mark scheme
both rely on metal ions giving out light of characteristic colours / wavelengths when heated in a flame
flame emission spectroscopy uses an instrument to detect and measure the exact wavelengths and intensities present (allowing mixtures to be identified and concentrations to be measured), rather than relying on the human eye judging an overall colour