AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical analysis › Identification of ions
Practise Flame tests. 10 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 tests identify some metal ions from the colour they give to a flame. This is in GCSE Chemistry only (not Combined Science) and is part of the required practical on identifying ions: learn the five colours, the method and why mixtures cause problems.
Key facts
Li+: crimson
Na+: yellow
K+: lilac
Ca2+: orange-red
Cu2+: green
Method: clean loop (acid + flame) → dip in sample → hot blue Bunsen flame → record colour
Mixtures: one colour can mask another (sodium's yellow hides others)
Notes
The flame colours
Flame tests identify some metal ions (cations).
Lithium compounds: crimson flame.
Sodium compounds: yellow flame.
Potassium compounds: lilac flame.
Calcium compounds: orange-red flame.
Copper compounds: green flame.
The colour comes from the metal ion, so sodium chloride and sodium sulfate both give a yellow flame. Some metal ions, such as magnesium, give no distinctive colour.
Method
diagram
Clean a nichrome wire loop: dip it in hydrochloric acid, then hold it in the flame until it gives no colour.
Dip the loop into the solid sample or its solution.
Hold the loop at the edge of a hot, blue Bunsen burner flame and record the colour. The yellow safety flame would hide the colours.
Clean loop, sample on the loop, edge of a blue flame; then read the colour.
Clean the loop again before testing the next sample.
Limitations
If a sample contains a mixture of metal ions, some flame colours can be masked (hidden). The strong yellow of sodium hides other colours, such as the lilac of potassium.
A flame test shows which metal ion is present, not how much of it there is.
Judging a colour by eye can be difficult, and a faint colour is easy to miss.
How to answer each type of question
Identify the metal ion from a flame colour
1 to 3 marksGrade 3
Learn the five colours using the exact spec words.
Match each colour in the question to its metal ion.
Name the ion, e.g. 'potassium ions' or K+.
Example. A student did flame tests on three white solids.
Solid
Flame colour
1
lilac
2
crimson
3
orange-red
Name the metal ion in each solid.Show the model answerHide the model answer
Clean the wire loop (hydrochloric acid, then the flame).
Dip the loop into the sample.
Hold it in a hot, blue Bunsen flame.
Observe and record the colour of the flame.
Example. Describe how a student could carry out a flame test on a solid compound.
Show the model answerHide the model answer
Dip a nichrome wire loop in hydrochloric acid and hold it in a flame until it gives no colour, to clean it (1). Dip the loop into the solid (1). Hold the loop in a hot, blue Bunsen burner flame (1). Record the colour of the flame (1).
Don’t lose marks
Using vague colours such as 'red', 'purple' or 'blue'. Use the spec words: crimson, yellow, lilac, orange-red, green.
Not cleaning the loop, or using the yellow safety flame, which hides the colours.
Saying the colour comes from the chloride or sulfate ion. The flame colour identifies the metal ion.
Assuming a yellow flame means sodium is the only metal ion present: other colours may be masked.
More tips
Memory tricks
Picture them: sodium street lamps glow yellow; old copper roofs turn green; Potassium = Pale Purple (lilac); Calcium = Carrot (orange-red); Lithium = Lipstick (crimson).
Crimson (lithium) is a deep red; orange-red is calcium. Never write just 'red'.
If a question mentions a mixture of metal ions, think 'masking'.
Exam technique
Name the ion, not just the element: 'lithium ions' or Li+.
In method questions, include cleaning the loop and using a hot, blue flame.
For 'identify the compound' questions, use the flame test for the metal ion and another test (carbonate, halide or sulfate) for the negative ion.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Recall the five flame test coloursLithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green.
Grade 4
Identify a metal ion from a flame colourMatch the colour seen to the metal ion that gives it.
Grade 4
Describe how to carry out a flame testDip a clean wire loop in the sample and hold it in a hot, blue Bunsen flame.
Grade 5
Explain why the wire loop must be cleanTraces of other metal ions would add their own colour and give a false result.
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.
Flame tests can be used to identify some metal ions. What colour flame is produced by potassium compounds?
lilac
One compound produced a lilac flame. Name the metal ion in this compound.
potassium
Which metal compound should be added to the firework mixture?
a potassium compound
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Flame tests can be used to identify some metal ions.
(a) What colour flame is produced by potassium compounds?[1]
(b) Which metal ion gives a crimson flame? Tick (✓) one box.[1]
Calcium
Copper
Lithium
Sodium
(c) A compound gives a green flame. Name the metal ion in the compound.[1]
(d) What colour flame is produced by calcium compounds?[1]
Show the answer and mark scheme
(a)Answer: lilac
lilac
(b)Answer: Lithium
(c)Answer: copper(II)
copper(II) / copper / Cu2+
(d)Answer: orange-red
orange-red
Question 2Medium5 marks
A student carried out flame tests on four solid compounds using a wire loop and a Bunsen burner.
(a) Describe how the student should carry out a flame test on one of the solid compounds.[3]
(b) Explain why the wire loop must be cleaned before each test.[1]
(c) One compound produced a lilac flame. Name the metal ion in this compound.[1]
Show the answer and mark scheme
(a)
clean the wire loop by dipping it in (hydrochloric) acid and holding it in the flame (until there is no colour)
dip the loop into the solid (moistened with acid)
hold the loop in the (edge of a) blue / roaring Bunsen burner flame and record the colour
(b)
to remove traces of the previous compound, which would affect the flame colour