Identification of Ions & Gases

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Identification of Ions & Gases

Every ion and gas has its own unique chemical "fingerprint" — a characteristic observable change when tested. Learn to recognise them.

What You'll Master

Testing for 5 anions (negative ions)
Testing for 8 cations (positive ions)
Identifying 6 gases by their reactions
Distinguishing between similar ions
Writing balanced ionic equations
Avoiding common exam mistakes

Identification of Anions (Negative Ions)

Anions are negatively charged non-metal ions. Each responds to a specific test with an observable change.

Carbonate Ions (CO₃²⁻)

What happens: Carbonates fizz when acid is added because they release CO₂ gas. That gas is then detected using limewater.

Step-by-step procedure:
1. Add dilute acid (e.g., dilute HCl) to the sample
2. You should see fizzing/effervescence
3. Collect the gas released
4. Bubble the gas through limewater (calcium hydroxide solution)
5. If carbonate is present, limewater turns cloudy/milky white

Positive result: Limewater turns cloudy white (or milky). This cloudiness is caused by a white precipitate of calcium carbonate forming.

CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

Exam tip: Don't just say "limewater turns cloudy." Examiners want to see that you know the full test: bubble the gas through limewater, which then turns cloudy. Simply observing cloudiness is just stating the result, not describing the test.

✓ Try This

A student adds dilute hydrochloric acid to a white powder and observes vigorous fizzing. She then bubbles the gas produced through limewater, which turns cloudy. What ion is present? Explain your answer in full.

Halide Ions (Cl⁻, Br⁻, I⁻)

What happens: Halide ions react with silver ions to form coloured silver halide precipitates. The colour depends on which halide it is — this is how we identify them.

Step-by-step procedure:
1. Acidify the sample with dilute nitric acid (HNO₃) — this is crucial, see the tip below
2. Add a few drops of silver nitrate solution (AgNO₃)
3. A silver halide precipitate forms if a halide ion is present
4. Observe and note the colour of the precipitate

Halide Ion	Precipitate Colour	Formula
Chloride (Cl⁻)	White	AgCl(s)
Bromide (Br⁻)	Cream	AgBr(s)
Iodide (I⁻)	Yellow	AgI(s)

Example: Cl⁻(aq) + Ag⁺(aq) → AgCl(s)

Critical:

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Also in the full note

Identification of Cations (Positive Ions)
Identification of Gases
What to Memorise
Concepts Checklist
Exam-Style Practice
Nitrate Ions (NO₃⁻)
Sulfate Ions (SO₄²⁻)
Sulfite Ions (SO₃²⁻)

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Identification of Ions & Gases