⭐⭐⭐⭐⭐ Very high for practical and observation questions.
Typical marks
5–15 marks depending on the question.
2. Aim
To identify unknown anions in solution by adding suitable reagents and observing characteristic results such as precipitates, effervescence, or colour changes.
3. Quick Theory
Anions are negatively charged ions. In qualitative analysis, we identify anions by adding reagents that produce a visible observation. The observation must be linked to a correct conclusion.
Example:
If dilute acid is added to a carbonate and effervescence occurs, carbon dioxide is produced. This suggests carbonate ions are present.
Examiner Tip: In Section A, the examiner often asks for both the test and the observation. Do not write the ion only.
4. Apparatus and Chemicals
Apparatus
Chemicals/Reagents
Test tubes, test tube rack, droppers, stirring rod, safety goggles.
Safety: Wear goggles. Silver nitrate can stain skin. Concentrated sulfuric acid is corrosive and should only be handled with great care under teacher supervision.
5. Labelled Experiment Diagram
Anion Analysis Setup
6. Procedure
A. Test for Carbonate Ions, CO3²⁻
Place a small amount of the unknown solution in a test tube.
Add dilute hydrochloric acid.
Observe whether effervescence occurs.
If gas is produced, test it with limewater if required.
B. Test for Sulfate Ions, SO4²⁻
Acidify the sample with dilute hydrochloric acid.
Add barium chloride solution.
A white precipitate indicates sulfate ions.
C. Test for Halide Ions, Cl⁻, Br⁻, I⁻
Acidify the sample with dilute nitric acid.
Add silver nitrate solution.
Observe the colour of the precipitate.
D. Test for Nitrate Ions, NO3⁻
Add freshly prepared iron(II) sulfate solution to the sample.
Carefully pour concentrated sulfuric acid down the side of the test tube.
A brown ring at the boundary indicates nitrate ions.
7. Observation Table
Anion
Test/Reagent
Positive Observation
Conclusion
Carbonate, CO3²⁻
Add dilute acid
Effervescence; CO2 turns limewater milky
Carbonate present
Sulfate, SO4²⁻
Acidify, then add BaCl2
White precipitate
Sulfate present
Chloride, Cl⁻
Acidify with HNO3, add AgNO3
White precipitate
Chloride present
Bromide, Br⁻
Acidify with HNO3, add AgNO3
Cream precipitate
Bromide present
Iodide, I⁻
Acidify with HNO3, add AgNO3
Yellow precipitate
Iodide present
Nitrate, NO3⁻
Brown ring test
Brown ring forms
Nitrate present
8. Why Acidification Is Important
Acidification removes interfering ions that may otherwise give false precipitates. For halide tests with silver nitrate, nitric acid is used because it does not introduce chloride ions.
Examiner Trap: Do not acidify halide tests with hydrochloric acid. Hydrochloric acid contains chloride ions, which can give a false positive result with silver nitrate.
9. Sources of Error and Improvements
Source of error
Effect
Improvement
Contaminated test tubes
False precipitate or wrong colour.
Use clean test tubes and droppers.
Too much reagent added
Observation becomes difficult to judge.
Add reagents dropwise.
Using wrong acid
False positive result.
Use the correct acid for each test.
Confusing precipitate colours
Wrong ion identified.
Compare with known positive controls.
10. Student Common Mistakes
Using hydrochloric acid before adding silver nitrate for halide tests.
Mixing up cream precipitate for bromide and yellow precipitate for iodide.
Writing “white solid” instead of “white precipitate”.
Saying carbonate produces oxygen instead of carbon dioxide.
Forgetting that sulfate also gives a white precipitate, but with barium chloride.
Confusing chloride white precipitate with sulfate white precipitate because both are white.
Writing the conclusion without the observation.
For nitrate, forgetting the brown ring test.
11. Examiner Tips
Tip 1: Always write reagent + observation + conclusion.
Tip 2: Use the phrase “white precipitate”, not just “white colour”.
Tip 3: Carbonate test often needs the gas test: carbon dioxide turns limewater milky.
Tip 4: For halides, the precipitate colour is the key mark.
12. Examiner Traps
Trap: Chloride and sulfate both produce white precipitates, but with different reagents.
Trap: Hydrochloric acid should not be used before silver nitrate in halide tests because it adds chloride ions.
Trap: Effervescence alone suggests a gas, but for carbonate the gas should be carbon dioxide.
13. 🎮 Interactive Simulator: Chemistry Detective
Unknown Sample
You have been given an unknown solution. Choose a test reagent, observe the result, then identify the anion.
Current sample:Unknown Sample A
Choose Reagent/Test
Identify the Anion
Observation Window
🫧🫧
No test yet
Choose a reagent and perform the test.
Detective Score: 0/0
Simulator Notes
Some tests give no visible result unless the correct reagent is used.
One unknown sample is active at a time.
Use your observation before choosing the final answer.
Wrong choices teach examiner traps, so mistakes are useful here.
14. Past Paper Style Questions
A student adds dilute hydrochloric acid to an unknown salt and observes effervescence. The gas turns limewater milky. Identify the anion. [3 marks]
Describe a test for sulfate ions and give the positive result. [3 marks]
Silver nitrate gives a cream precipitate with an acidified sample. Identify the anion. [2 marks]
Explain why nitric acid is used before adding silver nitrate in the test for halides. [3 marks]
A brown ring forms when a sample is tested with iron(II) sulfate and concentrated sulfuric acid. Identify the anion. [2 marks]
Q4. Nitric acid acidifies the solution [1]; removes interfering ions / prevents false results [1]; hydrochloric acid would add chloride ions and give a false positive with silver nitrate [1].
Q5. Nitrate ion / NO3- [2].
16. Quick Revision Card
Ion
Memory Hook
Carbonate
Acid makes bubbles. CO2 turns limewater milky.
Sulfate
Barium chloride gives white precipitate.
Chloride
Silver nitrate gives white precipitate.
Bromide
Silver nitrate gives cream precipitate.
Iodide
Silver nitrate gives yellow precipitate.
Nitrate
Brown ring test gives brown ring.
17. Mastery Checklist
I can state the aim of anion analysis.
I can identify carbonate using dilute acid.
I can identify sulfate using barium chloride.
I can identify chloride, bromide and iodide using silver nitrate.
I know why nitric acid is used before silver nitrate.
I can describe the brown ring test for nitrate.
I can write reagent, observation and conclusion in exam answers.
I can avoid the common precipitate colour mistakes.