Identifying an unknown from observations
Key idea: Combine test results to rule out ions that do not fit. If several candidates remain, choose a fresh-portion test that gives different results for them.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Choose the statement that records what you saw.
- Use each result to choose the next test.
- Follow how two results narrow seven cations down to two.
- Count the remaining cations and choose the next test.
- Read the two ions that can give ammonia, and how to check.
- Check where the ammonia came from.
- Decide which ions the two ammonia results show.
- Decide which chloride result you can trust.
- Explain which result to trust.
- Rule out one salt and choose the next test.
- Name the salt each result points to.
- Plan the tests without ammonia, then compare your plan with the model.
- Name the ions found and what remains unknown.
- Name the ions found and one that could remain.
- Read the common mistakes before you move on.
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ContinueAn unknown is identified by combining the results of several tests, cation, anion and gas, and ruling out every ion that does not fit. When more than one ion is left, the next step is the one test that would tell them apart.
Separate observations from conclusions
Check your understanding 1 What did you observe?
You add sodium hydroxide to an unknown solution. Which statement records an observation rather than an ion?
Show answer
Narrow down the possible ions and choose the next test
Start with the list of ions the unknown could contain. Each result rules out the ions that would have given a different result.
- If only one ion is left, you have identified it.
- If more than one is left, the results fit each of them. Name them all.
- The best next test is one that gives a different result for each ion that is left. Repeating a test that they all pass tells you nothing new.
Use a fresh portion of the unknown for every test, so that a reagent added earlier cannot cause the next result.
Worked example 1
Narrow down, then decide
Problem
A colourless solution X contains one of the seven cations you test for. A fresh portion gives a white precipitate with a few drops of aqueous sodium hydroxide, and the precipitate dissolves in excess to give a colourless solution. What can you conclude, and which one further test would decide the cation?
Show solution step by step
Rule out by colour
Method
Rule out every cation that gives a coloured precipitate or none.
Reason
Copper(II), iron(II) and iron(III) give coloured precipitates, and ammonium gives none.
Working
Answer
Left: aluminium, calcium, zinc.Rule out by the excess result
Method
Use what happened in excess sodium hydroxide.Reason
Calcium’s precipitate does not dissolve in excess, but this one did.
Working
Answer
Left: aluminium or zinc. The results fit both, so you cannot name the cation yet.
Choose the deciding test
Working
Answer
Add aqueous ammonia to a fresh portion, a few drops and then in excess. If the white precipitate stays, X contains aluminium ions; if it dissolves, X contains zinc ions.
Quick check
Repeating the sodium hydroxide test would not help: aluminium and zinc give the same result with it.
Check your understanding 2 Narrow it down in two stages
X contains one of the seven cations you test for. Sodium hydroxide gives a white precipitate that dissolves in excess. How many cations could it be after the white precipitate, and after it dissolves in excess? Which test tells the rest apart?
Show answer
A white precipitate leaves aluminium, calcium and zinc. Dissolving in excess rules out calcium, leaving aluminium or zinc. Add aqueous ammonia to a fresh portion, a few drops then in excess: the precipitate stays for aluminium and dissolves for zinc.
Check where ammonia could come from
Two different ions can give ammonia with warm sodium hydroxide. Ammonium ions give it with sodium hydroxide alone. Nitrate ions give it only when aluminium foil is added as well. So damp red litmus turning blue in the nitrate test could come from either ion.
To check, warm a fresh portion with sodium hydroxide without aluminium. If ammonia is given off, ammonium ions are present, and the nitrate test cannot tell you whether nitrate is there too. If no ammonia is given off, the ammonia in the nitrate test came from nitrate ions. Test the gas with damp red litmus paper; never smell it.
Guided practice 2
Where did the ammonia come from?
Problem
A student warms solution Y with aqueous sodium hydroxide and aluminium foil. Damp red litmus paper held at the mouth of the tube turns blue, and the student concludes that Y contains nitrate ions. What else could have given the ammonia, how should the student check, and what would each result of the check show?
Find where the gas came from
Hints
Hint 1
Same gas
Does this test also give ammonia without aluminium?
Hint 2
The check
What would you see if the sodium hydroxide alone, without the aluminium, gave the ammonia?
Show solution step by step
Name the second source
Reason
Ammonium ions give ammonia with warm sodium hydroxide, without any aluminium.
Working
Answer
The blue litmus fits nitrate ions, ammonium ions or both.
Check on a fresh portion
Working
Answer
Warm a fresh portion of Y with sodium hydroxide only. If no ammonia is given off here, but ammonia is given off once aluminium is added, Y contains nitrate ions.
Quick check
If sodium hydroxide alone also gives ammonia, the first result no longer shows nitrate ions: ammonium ions could explain all of it.
Check your understanding 3 Ammonia both times
Solution W gives ammonia when it is warmed with sodium hydroxide alone, and again when it is warmed with sodium hydroxide and aluminium foil. Which ions do these results show?
Show answer
Ammonium ions are present: sodium hydroxide alone gave ammonia. That ammonium also explains the ammonia in the second test, so these results cannot show whether nitrate ions are present as well.
Check whether a reagent caused the result
Hydrochloric acid adds chloride ions. If you add it before silver nitrate, a white precipitate can form even when the sample has no chloride. Use dilute nitric acid instead, then aqueous silver nitrate, on a fresh portion. A white precipitate then shows chloride; no precipitate means there is no chloride.
Check your understanding 4 Which result can you trust?
Portion 1: dilute nitric acid, then silver nitrate: no precipitate. Portion 2: dilute hydrochloric acid, then silver nitrate: white precipitate. Which result can you trust, and does the sample contain chloride ions?
Show answer
Trust portion 1: it used nitric acid, which adds no chloride, and it gave no precipitate, so the sample has no chloride. The hydrochloric acid in portion 2 added chloride ions, so that precipitate tells you nothing about the sample.
Spot the mistake 3
Two results that disagree
Learner record
A learner tests solution Z for chloride ions twice.
| Portion | Acid added | Then added | Result |
|---|---|---|---|
| 1 | dilute nitric acid | aqueous silver nitrate | no precipitate |
| 2 | dilute hydrochloric acid | aqueous silver nitrate | white precipitate |
The learner concludes that Z contains chloride ions because one test was positive. Explain to the learner which result to trust and why.
Decide which result counts
Show solution step by step
Check what each acid adds
Method
Ask whether anything added could itself cause the result.Reason
Hydrochloric acid contains chloride ions, which give a white precipitate with silver nitrate whatever is in the sample.Working
Answer
Portion 2’s precipitate may come from the acid, so it tells you nothing about Z.Conclude
Working
Answer
Portion 1 used the right acid and gave no precipitate, so Z contains no chloride ions. The precipitate in portion 2 came from the hydrochloric acid.
Common mistake
When two ions are left, write that the results “fit aluminium or zinc ions”. Write that they “show zinc ions” only when every other ion has been ruled out.
Plan a sequence of tests
Check your understanding 5 Use the clue before adding a test
A white solid is one of calcium carbonate, zinc sulfate, zinc chloride or aluminium sulfate, and it dissolves in water. Which salt can you rule out straight away, and which reagent tells aluminium from zinc?
Show answer
Calcium carbonate, because it does not dissolve in water. Aqueous ammonia, a few drops then in excess, tells aluminium from zinc; sodium hydroxide gives the same result for both. You could also test the anion first.
Check your understanding 6 Follow each result to a salt
For the three soluble salts, aqueous ammonia is added to a fresh portion until it is in excess. If the precipitate dissolves, another fresh portion is tested with dilute nitric acid, then barium nitrate. Which salt does each result point to?
Show answer
If the precipitate stays in excess ammonia, the salt is aluminium sulfate. If it dissolves, the salt is one of the two zinc salts: a white precipitate with barium nitrate means zinc sulfate, and no precipitate means zinc chloride. Acidified silver nitrate would confirm the chloride.
Try it yourself 4
Plan the tests yourself
Problem
A white solid is one of calcium carbonate, zinc sulfate, zinc chloride or aluminium sulfate, and it dissolves in water. You have dilute nitric acid, aqueous barium nitrate, aqueous silver nitrate and aqueous sodium hydroxide, but no aqueous ammonia. Plan tests on fresh portions to narrow it down as far as you can. Which salt can you identify, which two can you not tell apart, and which extra reagent would you ask for?
Write your plan
Hints
Hint 1
use solubility
Rule out the insoluble salt before adding any reagent.
Hint 2
the pair left
Compare the sodium hydroxide results for aluminium and zinc.
Show solution step by step
Use solubility
Reason
Calcium carbonate is insoluble in water.Working
Answer
Left: zinc sulfate, zinc chloride and aluminium sulfate.
Test the anion
Method
Add dilute nitric acid, then aqueous barium nitrate, to a fresh portion.
Reason
Only the two sulfates give a white precipitate.
Working
Answer
No precipitate: zinc chloride. A white precipitate: zinc sulfate or aluminium sulfate. Acidified silver nitrate would also pick out the chloride.
See what you cannot decide
Working
Answer
Sodium hydroxide gives a white precipitate that dissolves in excess for both aluminium and zinc, so it cannot tell the two sulfates apart. Ask for aqueous ammonia and add it to a fresh portion, a few drops then in excess: the precipitate stays for aluminium sulfate and dissolves for zinc sulfate.
Quick check
Check your plan for four things: it uses solubility, separates chloride from sulfate, recognises that sodium hydroxide cannot separate the two sulfates, and asks for ammonia to decide.
Check your understanding 7 What the results show
An unknown solution gives a red-brown precipitate with aqueous sodium hydroxide, insoluble in excess. Acidified silver nitrate gives a white precipitate. Which ions are present? Is there anything the results cannot tell you?
Show answer
Iron(III) ions, from the red-brown precipitate, and chloride ions, from the white precipitate with acidified silver nitrate. The results cannot tell you whether other anions, such as sulfate or nitrate, are also present, because no test for them was done.
Check your understanding 8 Three results on one solution
Fresh portions of a green solution are tested. Aqueous sodium hydroxide gives a green precipitate, insoluble in excess. Dilute nitric acid, then aqueous barium nitrate, gives a white precipitate. Warming with aqueous sodium hydroxide gives no gas. Which ions are present, which are not, and which ion could still be present?
Show answer
The green precipitate shows iron(II) ions, and the white precipitate with barium nitrate shows sulfate ions. No ammonia on warming means there are no ammonium ions. Chloride and nitrate ions could still be present, because neither was tested.
Common mistakes
- Naming one ion when the results fit two.
- Choosing a further test that cannot tell the remaining ions apart.
- Forgetting that a reagent can cause a result itself, such as chloride ions added in hydrochloric acid.
- Recording an inference without the observation it rests on.
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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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