This topic is examined in Paper 1, Paper 2, Paper 3, Paper 4, Paper 5, and Paper 6.
Anions are negatively charged ions. To identify them, we use specific chemical tests that produce characteristic observations such as precipitates or gases. The following table covers all required anion tests.
| Anion | Test Procedure |
|---|
| Observation | |
Why dilute nitric acid? For halide and sulfate tests, dilute nitric acid is used to acidify the solution because it does not contain chloride or sulfate ions itself. Using hydrochloric acid would introduce chloride ions, giving a false positive for chloride. Using sulfuric acid would introduce sulfate ions, giving a false positive for sulfate.
Cations are positively charged metal ions or ammonium. We identify them by adding aqueous sodium hydroxide (NaOH) or aqueous ammonia (NH_3) dropwise, then in excess. The colour of the precipitate and its solubility in excess reagent are key identifiers.
| Cation | Test with Aqueous Sodium Hydroxide ($NaOH$) |
|---|
| Test with Aqueous Ammonia (NH_3) | |
Note on Iron(II) vs Chromium(III): Both form green precipitates with sodium hydroxide. However, the chromium(III) precipitate dissolves in excess to form a dark green solution, whereas the iron(II) precipitate remains insoluble.
A flame test identifies metal ions by the characteristic colour they impart to a Bunsen burner flame. This occurs because electrons in the metal ions absorb energy, jump to higher energy levels, and then fall back down, emitting light of specific wavelengths.
Procedure: Clean a platinum or nichrome wire loop by dipping it in dilute hydrochloric acid and heating it in a flame until no colour is produced. Dip the clean loop into the solid sample (or concentrated solution) and place it in the edge of a blue Bunsen flame.
Why use cobalt glass for potassium? Sodium impurities are common in laboratory chemicals and produce a strong yellow flame that can mask the lilac colour of potassium. Cobalt glass filters out the yellow light, allowing the lilac colour to be seen.
Gases produced during reactions are identified by their specific chemical tests and observations.
A precipitate is an insoluble solid that forms and separates from a solution during a chemical reaction. In exam answers, you must state the colour of the precipitate (e.g., 'white precipitate') to gain marks.
When a test requires the solution to be 'acidified', it means dilute acid (usually nitric acid for halide/sulfate tests) is added first. This ensures that interfering ions (like carbonates or hydroxides) are removed before the main reagent is added.
Scenario: A solid salt X gives a brick-red flame in a flame test. When dilute nitric acid is added to a solution of X, no gas is produced. When aqueous silver nitrate is added to the acidified solution of X, a white precipitate forms.
Analysis:
- Flame test: Brick-red indicates Calcium ions (Ca^{2+}).
- Acid test: No gas with acid rules out carbonates (CO_3^{2-}) and sulfites (SO_3^{2-}).
- Silver nitrate test: White precipitate with acidified silver nitrate indicates Chloride ions (Cl^-).
Conclusion: Salt X is Calcium chloride (CaCl_2).
Mistake: Using hydrochloric acid to acidify the solution for the halide test.
Correction: You must use dilute nitric acid. Hydrochloric acid contains chloride ions (Cl^-). If you add it, you will get a white precipitate with silver nitrate regardless of whether the original sample contained chloride ions or not. This leads to a false positive result.
Mistake: Stating 'a gas is produced' as the observation for a test.
Correction: 'A gas is produced' is an inference, not an observation. You must describe what you see. For example, for carbonate ions, say 'effervescence occurs' or 'bubbles are seen'. For ammonia, say 'damp red litmus paper turns blue'. For hydrogen, say 'a squeaky pop is heard'.
Mistake: Using silver nitrate to test for chlorine gas, or using litmus paper to test for chloride ions.
Correction:
- Chloride ions (Cl^-) in solution are tested with acidified silver nitrate (white ppt).
- Chlorine gas (Cl_2) is tested with damp litmus paper (bleaches it). Do not confuse the two.
When to use: When describing tests for cations or halides.
Why examiners accept this: Examiners look for specific keywords: the reagent added, the action taken (e.g., 'dropwise', 'in excess'), and the precise observation including colour. Vague terms like 'a solid forms' are often rejected.
Correct Usage Example: Instead of saying 'It turns white,' write: 'A white precipitate forms.' If testing for aluminium ions, add: 'The precipitate is soluble in excess sodium hydroxide.'
When to use: When describing the test for ammonium ions or nitrate ions (via reduction).
Why examiners accept this: Ammonia is alkaline. The key indicator of an alkaline gas in Cambridge exams is the colour change of litmus paper. Mentioning the smell is good but the litmus change is the definitive chemical test.
Correct Usage Example: 'Damp red litmus paper turns blue.' Note: Do not say 'litmus paper turns blue' without specifying it was red initially, as litmus can be purple (neutral) or blue (alkaline).
When to use: When describing a flame test or identifying potassium.
Why examiners accept this: Sodium contamination is a major source of error. Examiners expect you to know that cobalt glass filters out the intense yellow sodium emission, allowing the fainter lilac potassium emission to be seen.
Correct Usage Example: 'The flame is lilac. If sodium contamination is suspected, view through cobalt blue glass to confirm the lilac colour.'
Q:A solution gives a white precipitate with aqueous sodium hydroxide, which is soluble in excess. It also gives a cream precipitate with acidified aqueous silver nitrate. Identify the cation and anion.
A:Cation: Aluminium (Al^{3+}) (white ppt soluble in excess NaOH). Anion: Bromide (Br^-) (cream ppt with AgNO_3).
Q:Describe the test for sulfate ions (SO_4^{2-}) and the expected observation.
A:Add dilute nitric acid followed by aqueous barium nitrate. A white precipitate forms.
Q:A gas turns damp red litmus paper blue. What is the gas?
Q:What flame colour is observed for calcium ions?
Q:Describe the test for oxygen gas.
A:Insert a glowing splint into the gas. The glowing splint relights.
Q:A solid gives a lilac flame in a flame test. Which ion is present?