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Qualitative Analysis: Gas Tests

The six K324 / 6092 gas tests for ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide, with exact observations and equations.

  • SEC G3 Pure Chemistry 2027

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Gas-test answers need two parts: the named test and its positive observation. Do not substitute a gas property such as smell for a chemical test.

1. Definition

A gas test uses a named procedure and positive observation to identify a gas. The inference is valid only when both parts match the prescribed test.

2. Key Ideas

GasTestPositive observation
ammonia, NH₃hold damp red litmus paper in the gaspaper turns blue
carbon dioxide, CO₂bubble through limewaterlimewater turns milky; a white precipitate forms
chlorine, Cl₂expose damp litmus paper to the gaspaper is bleached
hydrogen, H₂apply a lighted splintgas burns with a pop
oxygen, O₂insert a glowing splintsplint relights
sulfur dioxide, SO₂pass into aqueous acidified potassium manganate(VII)purple solution turns colourless

These are the six gas tests required in this course. Tests for water are useful elsewhere in practical chemistry but are not part of this gas-test set.

3. Detailed Explanations

A. Observation before inference

RepresentationCarbon-dioxide example
macroscopic observationlimewater turns milky; a white precipitate forms
inferencecarbon dioxide is present
particle explanationdissolved carbon dioxide ultimately provides carbonate species that form insoluble calcium carbonate with calcium ions
symbolic equationCO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

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

Do not write “carbon dioxide is seen.” The gas name is the conclusion; milkiness is the visible evidence. Limewater is initially clear. Record the initial milkiness: passing a large excess of carbon dioxide can eventually make the precipitate dissolve, so looking only at the final liquid can miss the positive test.

B. Why litmus must be damp

Ammonia must dissolve in water on the paper to produce hydroxide ions. Chlorine also needs moisture for its bleaching action. Therefore write damp red litmus for ammonia and damp litmus for chlorine.

NH₃(aq) + H₂O(l) ⇌ NH₄ + (aq) + OH⁻(aq)

Use the prescribed sulfur dioxide reagent

For sulfur dioxide, state aqueous acidified potassium manganate(VII) and the observation purple to colourless. “It bleaches litmus” is not the required syllabus test.

C. Safe test technique

  • Carry out practical gas tests with your teacher’s supervision using small, prepared samples.
  • Keep test tubes pointed away from people.
  • Never identify ammonia, chlorine or sulfur dioxide by inhaling directly.
  • Keep the hydrogen sample small and use only the stated splint test.
  • Follow the preparation instructions for the sample. Gas mixed with displaced air may give a misleading result; do not improvise a collection or ignition procedure.

4. Common Mistakes

  • Hydrogen uses a lighted splint; oxygen uses a glowing splint.
  • Limewater turns milky; “turns white” is imprecise.
  • Ammonia turns damp red litmus blue.
  • Chlorine bleaches damp litmus paper; a colour change without bleaching is incomplete evidence.
  • Sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless.
  • A pungent smell is not a safe identification test.

5. Exam Tips

  • Memorise each test and its positive observation as one pair.
  • Use QA Method to structure observation → inference answers.
  • Connect carbon dioxide and ammonia evidence to Anion Tests and Cation Tests.
  • For apparatus and density/solubility decisions, revise Collection of Gases.

6. Worked Examples

Modelled example 1

distinguish hydrogen and oxygen

Core

Problem

Sample A relights a glowing splint. Sample B burns with a pop when a lighted splint is applied. Identify both gases.

Study the worked solution

  1. Interpret sample A

    Method

    Identify oxygen.

    Reason

    Relighting a glowing splint is the prescribed positive test for oxygen.

    Working

    A: O₂.

  2. Interpret sample B

    Method

    Identify hydrogen.

    Reason

    A pop with a lighted splint is the prescribed positive test for hydrogen.

    Working

    B: H₂.

Guided practice 2

ammonia from an ion test

About 5 min

Problem

Warming an unknown solution with aqueous sodium hydroxide produces a gas that turns damp red litmus paper blue. State the gas and the ion supported by the full test.

Separate gas identity from original ion

Gas
Ion in original solution

Hints

Hint 1: gas property
The gas is alkaline and changes damp red litmus to blue.
Hint 2: full procedure
Use the warming-with-aqueous-sodium-hydroxide context to infer the original ion.
View solution step by step

  1. Identify the gas

    Method

    Name ammonia.

    Reason

    Ammonia dissolves in moisture on the litmus and gives an alkaline result.

    Working

    Gas: NH₃.

  2. Infer the ion

    Method

    State that ammonium ions are supported.

    Reason

    Ammonium ions release ammonia when warmed with aqueous sodium hydroxide.

    Working

    Ion: NH₄ +.

Common misconception 3

sulfur dioxide

Find and correct the mistake

Learner claim

A colourless gas changes aqueous acidified potassium manganate(VII) from purple to colourless. A learner identifies chlorine because “both tests use colour changes”. Correct the gas identification.

Match the exact reagent observation

Gas

View solution step by step

  1. Use the full observation

    Method

    Match purple-to-colourless acidified manganate(VII) to sulfur dioxide.

    Reason

    Sulfur dioxide reduces manganate(VII) ions under the prescribed test conditions.

    Working

    SO₂: acidified KMnO₄ purple → colourless.

  2. Reject colour-only matching

    Method

    Do not identify chlorine.

    Reason

    A valid gas test requires the named reagent and direction of change, not merely the word “colour”.

    Working

    Gas: sulfur dioxide, SO₂(g).

Guided practice 4

carbonate evidence chain

About 6 min

Connect the observations to the gas and ion

A solid effervesces with dilute acid. The evolved gas turns limewater milky. State the observations and infer the gas and original ion.

Move from observation to gas to ion

Evolved gas
Supported original ion

Hints

Hint 1: limewater
Name the gas that forms a milky white precipitate with limewater.
Hint 2: original solid
Ask which ion releases that gas when dilute acid is added.
View solution step by step

  1. Record and identify the gas

    Method

    Record effervescence and limewater turning milky.

    Reason

    The milky result is the prescribed positive test for carbon dioxide.

    Working

    Gas: CO₂; white precipitate in limewater.

  2. Infer the original ion

    Method

    State that carbonate ions are supported.

    Reason

    Carbonates react with dilute acids to release carbon dioxide.

    Working

    Original solid contains CO₃²⁻.

7. Mind Stretchers

Mind stretcher 1: Separate two colourless gasesExtension

Two unlabelled gas samples are hydrogen and sulfur dioxide. Design a safe identification sequence that gives positive evidence for each sample.

Show Answer

Use the teacher’s prepared small samples and fresh portions for different tests. First test a portion of each sample with aqueous acidified potassium manganate(VII). The sample that changes it from purple to colourless is sulfur dioxide. The other is the hydrogen candidate from the two stated possibilities; confirm it with the supervised lighted-splint test, which gives a pop. Do not use “no colour change” alone as a general test for hydrogen, and do not identify either gas by smell.

8. Practise and check

Practise and check

Practise the six required gases, exact test wording, observations and linked ion deductions.

Practise and check qualitative analysis
Syllabus and review details

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