Tests for Anions

This lesson covers the chemical tests used to identify anions (negative ions) in compounds, as required by the AQA GCSE Chemistry specification (8.1.2). You need to know how to test for carbonates, halides (chlorides, bromides, and iodides), and sulfates. These tests, combined with the flame tests and NaOH tests for cations, allow you to fully identify an unknown ionic compound.

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What Are Anions?

Anions are negatively charged ions. They are formed when atoms gain electrons. In ionic compounds, the anion is the non-metal part of the compound.

Term	Definition	Example
Cation	A positively charged ion (metal)	Na\u207a, Ca\u00b2\u207a, Fe\u00b3\u207a
Anion	A negatively charged ion (non-metal)	Cl\u207b, Br\u207b, SO\u2084\u00b2\u207b, CO\u2083\u00b2\u207b

To fully identify an unknown ionic compound, you test for both the cation (using flame tests or NaOH precipitate tests) and the anion (using the tests in this lesson).

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Test for Carbonates (CO\u2083\u00b2\u207b)

Method

1. Add dilute hydrochloric acid (HCl) to the substance being tested.
2. If a gas is produced, test the gas by bubbling it through limewater (calcium hydroxide solution).

Positive Result

• The substance fizzes (effervesces) when acid is added.
• The gas turns the limewater milky (cloudy).
• This confirms the gas is carbon dioxide, which means the substance contains carbonate ions.

Equation

The general equation for a carbonate reacting with acid:

Metal carbonate + Acid \u2192 Salt + Water + Carbon dioxide

Example: CaCO\u2083 + 2HCl \u2192 CaCl\u2082 + H\u2082O + CO\u2082

Exam Tip: If a question asks you to test for carbonates, you must describe two steps: (1) add dilute acid and observe fizzing, and (2) test the gas with limewater. Simply saying "it fizzes" is not enough — you need to confirm that the gas is carbon dioxide by the limewater test. Many students lose marks by only describing step 1.

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Tests for Halide Ions (Cl\u207b, Br\u207b, I\u207b)

Halide ions are the ions of the halogens (Group 7 elements). The three halide ions you need to test for are chloride, bromide, and iodide.

Method

1. Dissolve the substance in water (if it is not already in solution).
2. Add a few drops of dilute nitric acid (HNO\u2083) — this removes any interfering ions (such as carbonates or hydroxides) that could give false precipitates.
3. Add a few drops of silver nitrate solution (AgNO\u2083).
4. Observe the colour of the precipitate formed.

Precipitate Colours

Halide Ion	Formula	Precipitate Formed	Colour of Precipitate
Chloride	Cl\u207b	Silver chloride (AgCl)	White
Bromide	Br\u207b	Silver bromide (AgBr)	Cream
Iodide	I\u207b	Silver iodide (AgI)	Yellow

Why Add Nitric Acid First?

Dilute nitric acid is added to remove interfering ions. For example:

• Carbonate ions (CO\u2083\u00b2\u207b) would react with silver nitrate to form a white precipitate of silver carbonate, giving a false positive for chloride.
• Hydroxide ions (OH\u207b) would form silver hydroxide (also white).
• The nitric acid reacts with these interfering ions, removing them from solution before the silver nitrate is added.

Ionic Equations

Reaction	Ionic Equation
Chloride test	Ag\u207a(aq) + Cl\u207b(aq) \u2192 AgCl(s) [white precipitate]
Bromide test	Ag\u207a(aq) + Br\u207b(aq) \u2192 AgBr(s) [cream precipitate]
Iodide test	Ag\u207a(aq) + I\u207b(aq) \u2192 AgI(s) [yellow precipitate]

Exam Tip: The colours of the silver halide precipitates follow a pattern that gets "darker" as you go down Group 7: white (chloride) \u2192 cream (bromide) \u2192 yellow (iodide). This pattern can help you remember the colours. Also remember: you must add nitric acid first, then silver nitrate — if you add them in the wrong order or use a different acid (such as hydrochloric acid), you will not get the correct result.

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Test for Sulfate Ions (SO\u2084\u00b2\u207b)

Method

1. Dissolve the substance in water (if not already in solution).
2. Add a few drops of dilute hydrochloric acid (HCl) — this removes any interfering carbonate ions.
3. Add a few drops of barium chloride solution (BaCl\u2082).
4. Observe whether a precipitate forms.

Positive Result

• A white precipitate of barium sulfate (BaSO\u2084) forms.
• This confirms the presence of sulfate ions.

Ionic Equation

Ba\u00b2\u207a(aq) + SO\u2084\u00b2\u207b(aq) \u2192 BaSO\u2084(s) [white precipitate]

Why Add Hydrochloric Acid First?

• Hydrochloric acid removes carbonate ions, which would otherwise react with barium chloride to form white barium carbonate — giving a false positive for sulfate.

Exam Tip: Do not confuse the halide test with the sulfate test. For halides, use nitric acid + silver nitrate. For sulfates, use hydrochloric acid + barium chloride. A common exam mistake is mixing up the reagents. Use this memory aid: "Silver for halides, Barium for sulfates" and "Nitric for halides, Hydrochloric for sulfates."

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Anion Test Decision Flowchart

flowchart TD
    A["Unknown Anion"] --> B{"Add dilute HCl"}
    B -->|"Fizzes, gas turns limewater milky"| C["Carbonate CO3 2-"]
    B -->|"No fizzing"| D{"Dissolve in water, add dilute HNO3, then AgNO3"}
    D -->|"White precipitate"| E["Chloride Cl-"]
    D -->|"Cream precipitate"| F["Bromide Br-"]
    D -->|"Yellow precipitate"| G["Iodide I-"]
    D -->|"No precipitate"| H{"Add dilute HCl, then BaCl2"}
    H -->|"White precipitate"| I["Sulfate SO4 2-"]
    H -->|"No precipitate"| J["None of the above anions"]

    style A fill:#2c3e50,color:#fff
    style C fill:#27ae60,color:#fff
    style E fill:#f5f5f5,color:#2c3e50
    style F fill:#f5deb3,color:#2c3e50
    style G fill:#f1c40f,color:#2c3e50
    style I fill:#ecf0f1,color:#2c3e50

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Summary Table: All Anion Tests