Experiment: Qualitative analysis of anions

Qualitative analysis of anions means finding out which negative ions are present in a salt. Each common anion has its own confirmatory test with a clear observation, so this is a favourite Paper 3 practical that rewards careful, ordered work. This guide sets out the tests for carbonate, chloride, sulfate and nitrate, the observations to expect, and the science process skills the practical papers reward. It pairs naturally with the identification of cations, so that a full salt can be named.

Aim

To identify the anion present in a given salt solution using specific confirmatory tests for carbonate, chloride, sulfate and nitrate ions.

Apparatus and materials

• Test tubes and a test tube rack
• Delivery tube and a test tube of limewater (for the carbonate test)
• Dropper (teat pipette)
• Bunsen burner
• Dilute hydrochloric acid and dilute nitric acid
• Barium chloride solution and silver nitrate solution
• Iron(II) sulfate solution and concentrated sulfuric acid (for the nitrate brown-ring test)
• The unknown salt solution

Procedure

1. Carbonate test. Add dilute acid to a portion of the solid or solution. If it fizzes, pass the gas through limewater and observe whether the limewater turns milky.
2. Sulfate test. To a fresh portion of the solution, add a little dilute hydrochloric acid to remove any carbonate, then add barium chloride solution and observe.
3. Chloride test. To a fresh portion, add a little dilute nitric acid, then add silver nitrate solution and observe; then add ammonia solution to test whether the precipitate dissolves.
4. Nitrate test (brown ring). To a fresh portion, add iron(II) sulfate solution, then carefully add concentrated sulfuric acid down the side of the tilted test tube so it forms a layer at the bottom, and observe the junction of the two liquids.
5. Record each observation next to the test, and match it to the known result to identify the anion.

Expected observations

In the carbonate test, adding dilute acid produces effervescence, and the gas given off turns limewater milky, showing carbon dioxide and therefore carbonate. In the sulfate test, a white precipitate that stays after acidifying confirms sulfate. In the chloride test, a white precipitate that dissolves in ammonia solution confirms chloride. In the nitrate brown-ring test, a brown ring forms where the concentrated sulfuric acid layer meets the solution above it, confirming nitrate. Recording exactly what is seen, and in the right order, is what makes the identification reliable.

Inference and conclusion

Each anion reacts in a characteristic way that produces a distinctive, observable result, a gas, a precipitate of a particular colour and solubility, or a coloured ring. By carrying out the correct test and matching the observation to the known outcome, the anion is identified. Acidifying before the sulfate and chloride tests is essential, because carbonate ions would otherwise form a white precipitate too and give a false positive. The conclusion names the anion present and quotes the confirmatory observation that supports it.

Science process skills (Paper 3 style)

Following a logical sequence. Explain why the carbonate test is done first and why acidifying comes before adding barium chloride or silver nitrate, to remove an interfering ion so the later test is valid.

Controlling variables. Use a fresh portion of solution for each test and the same procedure each time, so the observations can be compared fairly and are due to the anion present.

Making an operational definition. Define a positive sulfate test operationally as a white precipitate that remains after the solution has been acidified with dilute hydrochloric acid.

Inferring and communicating. From each observation, infer the anion, then write a conclusion that names the ion and states the test result that identifies it, in clear, precise language.

Safety precautions

• Wear safety goggles, because the acids and silver nitrate are corrosive or irritant and stain or burn.
• Add concentrated sulfuric acid slowly down the side of a tilted tube for the brown-ring test, because it releases heat and can spit if added carelessly.
• Do not let the gas from the carbonate test build up; direct it through the delivery tube into limewater, so fumes are not inhaled.
• Wash any silver nitrate off the skin, because it stains and is irritant, and wash hands after handling any toxic salts.

Common errors

• Not acidifying before the sulfate or chloride test. Carbonate would also give a white precipitate and a false positive; always acidify first.
• Using the wrong acid. Use dilute hydrochloric acid before the sulfate test and dilute nitric acid before the chloride test, so the added acid does not itself supply the ion being tested.
• Adding the concentrated acid too fast in the brown-ring test. Poured in quickly, it mixes and no ring forms; add it slowly to form a separate layer.
• Reusing a contaminated test tube. Traces of a previous reagent can give a misleading result; use a clean tube and a fresh portion each time.
• Vague recording. State the actual observation, effervescence, a white precipitate, a brown ring, not just “a positive result”.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we teach anion analysis as an ordered routine, carbonate first, then acidify, then the specific test, so that interfering ions are removed and each observation means what it should. Because this practical is a standard part of Paper 3 (Paper 3 is a practical test assessing science process skills) within SPM Chemistry, and because it combines with cation analysis to name a whole salt, the discipline of testing in the right order and recording exactly what is seen is where the marks are secured.