Carbonate and halide tests: reagent then observation
Connect carbonate gas confirmation and acidified silver-nitrate halide tests, keeping the test, observation and identification separate.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Interpret the supplied laboratory observations on paper; actual chemical tests require an appropriate supervised laboratory method. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Distinguish gas evolution from a precipitate.
- Read an ion formula and state an observation before an inference.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a precipitate?
What you will learn
- Describe the carbonate test with carbon-dioxide confirmation.
- Identify chloride, bromide and iodide from the required acidification and silver-nitrate observations.
Carbonate and halide 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
Carbonate · CO₃²⁻
- Test
- Dilute acid, then test the evolved gas with limewater.
- Observation
- Effervescence; carbon dioxide turns limewater milky.
Chloride · Cl⁻
- Test
- Dilute nitric acid, then aqueous silver nitrate.
- Observation
- White precipitate.
Bromide · Br⁻
- Test
- Dilute nitric acid, then aqueous silver nitrate.
- Observation
- Cream precipitate.
Iodide · I⁻
- Test
- Dilute nitric acid, then aqueous silver nitrate.
- Observation
- Yellow precipitate.
Use the supplied candidate set and conditions. One observation does not establish every ion in a mixture or certify purity.
Bubbles need a gas-confirmation test
For a carbonate in the supplied supervised-test model, adding dilute acid gives effervescence and carbon dioxide. Confirm the gas using limewater, which turns milky. Effervescence is the visible observation; carbon dioxide is the gas identification supported by the confirming test. Bubbles alone do not distinguish every possible gas-producing reaction.
Carbonate has formula CO₃²⁻. The gas formed is CO₂, not CO or oxygen. State reagent, observed response and confirming test in order. Laboratory observations can be affected by contamination and method, so these exercises use the stated candidate ions and controlled observations rather than assuming every unknown household mixture is suitable for testing.
Halides produce different silver-halide colours
For chloride Cl⁻, bromide Br⁻ or iodide I⁻ in solution, acidify with dilute nitric acid, then add aqueous silver nitrate, AgNO₃. The required precipitate colours are white silver chloride AgCl, cream silver bromide AgBr and yellow silver iodide AgI. A precipitate is an insoluble solid appearing in the liquid; it is not the same observation as a clear solution changing colour.
The acid choice matters: hydrochloric acid would introduce chloride and could cause a misleading silver-chloride observation. Use the specified nitric-acid pretreatment rather than treating all acids as interchangeable. The reference compares the required candidates under the supplied method. In a more complicated sample, one colour alone is not complete proof of every ion present; report what the stated test supports.
Pause and explain
Why is dilute nitric acid specified rather than hydrochloric acid for the silver-nitrate halide test?
Worked example
A supplied unknown gives a cream precipitate after dilute nitric acid and aqueous silver nitrate. Another portion releases a gas with dilute acid that turns limewater milky. Identify the supported anions.
Show the worked solution
- The acidified silver-nitrate cream precipitate matches bromide among the required halides.
- Effervescence followed by milky limewater confirms carbon dioxide in the stated carbonate test.
- The two observations support bromide and carbonate in the supplied candidate model, using separate portions.
Answer Bromide Br⁻ and carbonate CO₃²⁻ are supported by the respective stated observations.
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.
Describe the carbonate test, including confirmation of the gas.
Give me a hint
Separate initial bubbles from the identification test.
Compare my reasoning
- Add dilute acid in the stated supervised-test model and observe effervescence.
- Test the resulting gas with limewater.
- Milky limewater supports carbon dioxide and therefore carbonate in the supplied candidate scenario.
Dilute acid gives effervescence; the gas turns limewater milky, supporting carbonate CO₃²⁻.
Look for these in your work
- Reagent, visible response and gas test are all stated.
- Carbon dioxide is not inferred from bubbles alone.
Give the pretreatment, reagent and required chloride/bromide/iodide observations.
Give me a hint
Use the specified acid and match three distinct precipitate colours.
Compare my reasoning
- Acidify each suitable aqueous sample with dilute nitric acid.
- Add aqueous silver nitrate.
- White supports Cl⁻, cream supports Br⁻ and yellow supports I⁻ under the stated method.
Dilute nitric acid, then aqueous AgNO₃: chloride white, bromide cream, iodide yellow precipitate.
Look for these in your work
- The acid/reagent order is correct.
- All three halide observations are correctly assigned.
A student acidifies an unknown with hydrochloric acid, adds silver nitrate and sees a white precipitate. Can they confidently attribute chloride to the original sample?
Give me a hint
Check whether the method itself added the tested ion.
Compare my reasoning
- Hydrochloric acid adds chloride ions.
- Those introduced ions can give silver chloride with the silver-nitrate reagent.
- The observation cannot confidently establish chloride in the original sample; repeat an appropriate controlled method with the specified nitric-acid pretreatment.
No: the hydrochloric-acid pretreatment introduced chloride, so the original sample's chloride is not established by that result.
Look for these in your work
- The contamination mechanism is explained.
- The improved method removes the identified interference.
Common mistakes
- Calling every bubbling acid reaction proof of carbonate without testing the gas.
- Using hydrochloric acid before the halide silver-nitrate test.
- Swapping cream bromide and yellow iodide precipitates.
- Describing a precipitate only as a dissolved liquid colour.
What can you explain now?
Which halide gives a yellow precipitate in the stated acidified silver-nitrate test?
Compare with the explanation
Iodide, I⁻.
The required chloride/bromide/iodide precipitates are white/cream/yellow respectively.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall reagent, observation and inference for carbonate and each halide, then explain why a wrong pretreatment can create a misleading positive result.
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.