White cation precipitates and the separate ammonium gas test
Distinguish aluminium, zinc and calcium using both aqueous reagents and their excess behaviour, then identify ammonium by its specified gas test.
Core and Extended · Chemistry 0620, 2026–2028. Interpret the supplied laboratory observations on paper; actual chemical tests require an appropriate supervised laboratory method.
Before you begin
- Define precipitate and distinguish initial addition from excess reagent.
- Read cation formulas and recognise damp-red-litmus evidence for ammonia.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does 'soluble in excess' describe here?
What you will learn
- Compare required NaOH and ammonia observations for Al³⁺, Zn²⁺ and Ca²⁺.
- Describe the ammonium test without treating added ammonia as evidence from the unknown.
White cation and ammonium comparison reference
Try recalling the result, then open this comparison. Read the test and observation together. Actual testing belongs to an appropriate supervised laboratory method.
Open the comparison reference
Aluminium · Al³⁺
- Aqueous sodium hydroxide
- White precipitate; soluble in excess to a colourless solution.
- Aqueous ammonia
- White precipitate; insoluble in excess.
Ammonium · NH₄⁺
- Aqueous sodium hydroxide
- Ammonia is produced on warming; confirm with damp red litmus turning blue.
- Aqueous ammonia
- No identification observation is specified for this reagent in the required notes.
Calcium · Ca²⁺
- Aqueous sodium hydroxide
- White precipitate; insoluble in excess.
- Aqueous ammonia
- No precipitate or a very slight white precipitate.
Zinc · Zn²⁺
- Aqueous sodium hydroxide
- White precipitate; soluble in excess to a colourless solution.
- Aqueous ammonia
- White precipitate; soluble in excess to a colourless solution.
Use the supplied candidate set and conditions. One observation does not establish every ion in a mixture or certify purity.
White alone does not identify the cation
In the supplied tests, Al³⁺, Zn²⁺ and Ca²⁺ can each form a white precipitate with aqueous sodium hydroxide. Additions and excess behaviour distinguish them: aluminium's precipitate dissolves in excess NaOH to give a colourless solution; zinc's does too; calcium's remains insoluble in excess. A single white NaOH precipitate therefore does not uniquely identify one of these candidates.
Compare a separate suitable portion with aqueous ammonia. Aluminium gives a white precipitate insoluble in excess ammonia. Zinc gives a white precipitate soluble in excess ammonia, forming a colourless solution. Calcium gives no precipitate or a very slight white precipitate in the required notes. The calcium entry is deliberately qualified; do not convert it into a guarantee that exactly the same visible amount always forms.
Ammonium is tested through its released gas
Ammonium NH₄⁺ is different from ammonia NH₃. The required ammonium test adds aqueous sodium hydroxide and warms in the appropriate supervised method; ammonia is produced and turns damp red litmus blue. It is a gas observation, not one of the white-precipitate comparisons. Do not deliberately smell an unknown gas.
The qualitative-analysis notes do not provide an aqueous-ammonia cation-test observation for ammonium. A dash in that reference does not mean every conceivable reaction is impossible. It means the specified identification route here is the NaOH/warming gas test. Adding aqueous ammonia and then finding ammonia does not establish that it came from the original unknown.
Pause and explain
White precipitate: soluble in excess NaOH, insoluble in excess ammonia. Which stated candidate fits?
Worked example
Unknown W gives a white precipitate soluble in excess NaOH but insoluble in excess aqueous ammonia. Choose among aluminium, zinc and calcium.
Show the worked solution
- White with NaOH alone leaves several possible candidates.
- Solubility in excess NaOH rules out calcium in the supplied comparison.
- Insolubility in excess ammonia distinguishes aluminium from zinc.
Answer Aluminium Al³⁺ is the supported candidate under the stated two-reagent comparison.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare the initial white NaOH precipitates and their excess behaviour for Al³⁺, Zn²⁺ and Ca²⁺.
Give me a hint
Keep the same initial colour separate from different later behaviour.
Compare my reasoning
- All three give white precipitate in the supplied NaOH comparison.
- Aluminium and zinc dissolve in excess to give colourless solution.
- Calcium remains insoluble in excess; ammonia on separate portions helps distinguish the other two.
NaOH: Al³⁺ and Zn²⁺ white then soluble in excess; Ca²⁺ white and insoluble in excess.
Look for these in your work
- Initial and excess stages are separately stated.
- The result is not overclaimed as unique from colour alone.
Unknowns X, Y and Z fit these supplied patterns: X white/soluble in excess with both reagents; Y NaOH white/soluble but ammonia white/insoluble; Z NaOH white/insoluble and ammonia no or very slight white precipitate. Identify each.
Give me a hint
Compare the complete two-reagent patterns.
Compare my reasoning
- X fits zinc's solubility in excess of both reagents.
- Y fits aluminium's difference between excess NaOH and ammonia.
- Z fits calcium, including the qualified no-or-very-slight ammonia observation.
X Zn²⁺; Y Al³⁺; Z Ca²⁺ under the supplied candidate patterns.
Look for these in your work
- All three complete patterns are matched.
- The calcium qualification is retained.
A sample warmed with aqueous NaOH produces a gas turning damp red litmus blue. Another student instead adds ammonia reagent and calls the presence of ammonia proof of ammonium. Compare the evidence.
Give me a hint
Track whether the detected gas could have been introduced by the reagent.
Compare my reasoning
- The NaOH/warming observation supports ammonium in the supplied cation candidate set, with ammonia confirmed by litmus.
- Ammonia deliberately added as reagent is not proof that it originated from the unknown.
- Use the specified test and separate suitable portions rather than confusing NH₄⁺ with NH₃ reagent.
The stated NaOH/warming test supports NH₄⁺ in the candidate set; added NH₃ reagent cannot establish ammonium in the original sample.
Look for these in your work
- Ammonium and ammonia formulas are distinguished.
- Reagent-origin interference is explained.
Common mistakes
- Naming a cation from white precipitate alone.
- Assuming aluminium and zinc behave identically in excess ammonia.
- Replacing calcium's qualified ammonia observation with an absolute rule.
- Confusing ammonium in the sample with ammonia added as reagent.
What can you explain now?
Which of the white-precipitate candidates dissolves in excess of both reagents?
Compare with the explanation
Zinc, Zn²⁺.
Its white precipitate gives a colourless solution in excess NaOH and excess aqueous ammonia.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall both reagent patterns for aluminium, zinc and calcium, then explain why ammonium uses a separate released-gas test.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.