Cation tests: precipitates and excess reagent
Key idea: Identify metal cations by adding sodium hydroxide and ammonia to fresh portions, first dropwise then in excess. Both precipitate colour and whether it dissolves provide evidence.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Read how to record the colour and what happens in excess.
- Read how to do the tests safely on fresh portions.
- Follow how the results show copper(II).
- Decide what is light blue and what is dark blue.
- Use excess ammonia to tell the two white precipitates apart.
- Count the cations left, then choose the next test.
- Explain how you narrow it down, and why white alone cannot show zinc.
- Choose the test for ammonium ions and its result.
- Name both iron ions, then decide whether ammonia helps.
- Explain both results, and why ammonia adds nothing new here.
- Name the cation, and how it differs from aluminium and zinc.
- Write what you saw, then how each result rules out the others.
- Read the common mistakes before the topic check.
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ContinueMost metal cations form a precipitate with aqueous sodium hydroxide or aqueous ammonia. The colour of the precipitate, and whether it dissolves when more reagent is added, identify the ion.
An observation is what you see, such as “a white precipitate forms”. A precipitate is an insoluble solid that forms when two solutions are mixed. An inference is the identity the observations support, such as “zinc ions are present”. You see a light-blue solid; you infer copper(II) ions.
Use a few drops, then excess reagent
Take a fresh portion of the solution for each reagent. Add the reagent a few drops at a time, look, then keep adding until it is in excess. The two stages give two separate observations:
- the colour of any precipitate that forms with a few drops;
- whether that precipitate stays or dissolves when excess reagent is added, and the colour of any solution left.
A precipitate colour, a solution colour and the behaviour in excess are three different things to record.
Swipe or scroll sideways to inspect the complete overview.
| Cation | Aqueous sodium hydroxide, dropwise then in excess | Aqueous ammonia, dropwise then in excess |
|---|---|---|
| aluminium, Al³⁺ | white precipitate; dissolves in excess to give a colourless solution | white precipitate; insoluble in excess |
| ammonium, NH₄⁺ | no precipitate; on warming, ammonia is given off, which turns damp red litmus blue | not used |
| calcium, Ca²⁺ | white precipitate; insoluble in excess | no precipitate |
| copper(II), Cu²⁺ | light blue precipitate; insoluble in excess | light blue precipitate; dissolves in excess to give a dark blue solution |
| iron(II), Fe²⁺ | green precipitate; insoluble in excess | green precipitate; insoluble in excess |
| iron(III), Fe³⁺ | red-brown precipitate; insoluble in excess | red-brown precipitate; insoluble in excess |
| zinc, Zn²⁺ | white precipitate; dissolves in excess to give a colourless solution | white precipitate; dissolves in excess to give a colourless solution |
The coloured precipitates are metal hydroxides. For example, copper(II) ions and hydroxide ions form copper(II) hydroxide:
You do not need the formula of the dark blue substance that forms when the precipitate dissolves in excess ammonia.
Three cations give a white precipitate with sodium hydroxide, so “white precipitate” alone never identifies an ion. Read down the table for the observation that separates them:
- calcium: the white precipitate stays in excess sodium hydroxide, and aqueous ammonia gives no precipitate;
- aluminium and zinc: both white precipitates dissolve in excess sodium hydroxide, so, among these two reagents, excess aqueous ammonia separates them: aluminium’s precipitate stays, zinc’s dissolves.
Practical task 1 Dropwise, then in excessOptional
Use about 1 cm³ of the solution in a clean test tube. Add the reagent a few drops at a time and shake gently; record the colour of any precipitate. Keep adding until no further change occurs and record whether the precipitate stays or dissolves. Use a fresh portion for the other reagent. Wear eye protection; aqueous sodium hydroxide is corrosive.
Worked example 1
From a results table to a cation
Problem
A student records these results for solution P.
| Test on a fresh portion of P | Few drops | In excess |
|---|---|---|
| aqueous sodium hydroxide | light blue precipitate | precipitate stays |
| aqueous ammonia | light blue precipitate | precipitate dissolves; dark blue solution |
Which cation does P contain? Give the evidence before the conclusion.
Show solution step by step
Start from the colour
Method
Match the precipitate colour first.Reason
Of the seven cations, only copper(II) gives a light blue precipitate.Working
Answer
Light blue precipitate with both reagents: consistent with Cu²⁺.Check the excess behaviour
Method
Confirm with the result in excess ammonia.Reason
A conclusion is stronger when every observation agrees with it, not just the first one.Working
Answer
Insoluble in excess NaOH(aq); dissolves in excess NH₃(aq) to a dark blue solution. Both match copper(II).State the inference
Working
Answer
Observations: light blue precipitate, insoluble in excess sodium hydroxide, dissolving in excess ammonia to a dark blue solution. Inference: P contains copper(II) ions.
Quick check
Check your understanding 1 Record which material has the colour
For copper(II) with aqueous ammonia, check the observation with a few drops and the observation in excess. Do not turn a solution colour into a precipitate colour.
Show answer
A few drops give a light blue precipitate. In excess ammonia it dissolves, leaving a dark blue solution. With sodium hydroxide the light blue precipitate remains in excess.
Guided practice 2
Two white precipitates
Problem
Solutions Q and R give identical results with aqueous sodium hydroxide: a white precipitate that dissolves in excess to give a colourless solution. With aqueous ammonia, Q gives a white precipitate that stays in excess; R gives a white precipitate that dissolves in excess. Identify the cation in each solution.
Decide from the evidence
Hints
Hint 1
Same first test
Which ions match the excess-sodium-hydroxide result in the table?Hint 2
Decisive test
Compare only the excess-ammonia results; that is the one observation that differs.Show solution step by step
Rule out calcium
Method
Use the excess sodium hydroxide result.Reason
Calcium’s white precipitate is insoluble in excess sodium hydroxide, but both precipitates here dissolved.Working
Answer
Possible cations: Al³⁺ or Zn²⁺ for both solutions.Use the decisive observation
Reason
Of these two reagents, only excess ammonia gives different results for the two cations.Working
Answer
Q: precipitate stays in excess ammonia, so aluminium ions. R: precipitate dissolves in excess ammonia, so zinc ions.
Quick check
Check a conclusion based only on colour
Check your understanding 2 Find the missing evidence
A few drops of aqueous sodium hydroxide give a white precipitate. How many of the seven cations could it be, and which results would narrow it down?
Show answer
Three cations are left: aluminium, calcium and zinc. Record excess sodium hydroxide behaviour first. If it dissolves, use excess aqueous ammonia on a fresh portion: aluminium’s precipitate stays, zinc’s dissolves.
Spot the mistake 3
“A white precipitate, so it is zinc”
Learner response
A learner adds aqueous sodium hydroxide to solution S and writes: “A white precipitate forms, so S contains zinc ions.” Explain why the learner cannot say this yet, and what the learner must still do.
Name the missing evidence
Show solution step by step
Find the gap
Method
Count the cations the result leaves.Reason
Aluminium, calcium and zinc all give a white precipitate with a few drops of sodium hydroxide.
Working
Answer
One colour word leaves three possible cations.
Add the missing observations
Working
Answer
Record whether the precipitate dissolves in excess sodium hydroxide. If it dissolves, test a fresh portion with aqueous ammonia and record whether its white precipitate dissolves in excess. Only a precipitate that dissolves in excess of both reagents supports zinc ions.
Common mistake
Naming an ion from the first colour alone treats a possibility as a result. The excess-reagent behaviour is part of the evidence, not an optional extra.
Write the reagent, the observation with a few drops, the observation in excess, and then the inference. When two cations give the same first result, the next observation must be one that tells them apart.
Check your understanding 3 Recognise ammonium without a precipitate
For a solution tested for ammonium under the supervised procedure, choose the warmed reagent and the condition and positive observation of the gas-test paper.
Show answer
Warm a fresh portion with aqueous sodium hydroxide. Ammonia is released and turns damp red litmus blue. No precipitate is required for this cation test. Test the gas at the tube mouth, not the liquid; do not smell gases directly. Practical work requires the supervised procedure.
Apply the tests to iron ions
Check your understanding 4 Choose from both iron observations
T gives a green precipitate and U gives a red-brown precipitate with sodium hydroxide; both are insoluble in excess. Identify both cations and decide whether ammonia gives an additional distinction.
Show answer
T contains iron(II) and U contains iron(III). Ammonia gives the same colours, and both stay insoluble in excess, so it only confirms what you already know.
Check your understanding 5 Which iron ion?
Two solutions each give a precipitate with aqueous sodium hydroxide that is insoluble in excess. The precipitate from T is green; the precipitate from U is red-brown. Identify the cation in each, and explain why adding aqueous ammonia would not distinguish the ions further.
Work on paper or use optional notes; explain every requested contrast before comparing the model.
Show answer
T contains iron(II) ions (green precipitate) and U contains iron(III) ions (red-brown precipitate). Aqueous ammonia gives the same colours and the same insolubility in excess, so it cannot separate them further; the colour is already the decisive observation. An ammonia test could still check that the observations agree with the proposed identities.
Check your understanding 6 Check both exclusions
V gives a white precipitate that stays in excess sodium hydroxide and no precipitate with ammonia on fresh portions. Check its identity and both contrasts with aluminium and zinc.
Show answer
V supports calcium. Aluminium and zinc both dissolve in excess sodium hydroxide and both give a white precipitate with ammonia, so either observed contrast rules them out.
Check your understanding 7 Explain both calcium evidence lines
A solution V contains one of the seven cations in this lesson’s table. On fresh portions, aqueous sodium hydroxide gives a white precipitate that stays in excess; aqueous ammonia gives no precipitate. State the observations and inference, and explain how each result rules out aluminium and zinc.
Work on paper or use optional notes; explain every requested contrast before comparing the model.
Show answer
Aqueous sodium hydroxide: white precipitate, insoluble in excess. Aqueous ammonia: no precipitate. Inference: V contains calcium ions, Ca²⁺. Either result alone rules out aluminium and zinc: their white precipitates dissolve in excess sodium hydroxide, and both form a white precipitate with aqueous ammonia.
Common mistakes
- Writing the ion as the observation, such as “copper ions form”, instead of the precipitate and its colour.
- Giving the colour of a precipitate without saying whether it dissolves in excess.
- Mixing up aluminium and zinc. Only the zinc precipitate dissolves in excess aqueous ammonia.
- Writing “dark-blue precipitate” for copper(II) in excess ammonia. The dark blue is a solution.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
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