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Tests for Carbon Dioxide, Hydrogen and Oxygen

Learn the prescribed tests and positive observations for carbon dioxide, hydrogen and oxygen.

  • SEC G2 Science Chemistry component 2027
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Learning objectives

  • describe tests to identify the following gases: carbon dioxide (using limewater); hydrogen (using a burning splint); oxygen (using a glowing splint).

This lesson covers the three gas tests prescribed for G2 Science (Chemistry): carbon dioxide, hydrogen and oxygen.

1. Definition

A gas test is a prescribed procedure that produces a characteristic positive observation used to identify a gas.

  • Observation: what is directly seen or heard.
  • Inference: the identity concluded from that observation.
Course boundary

Cation tests, anion tests and the gas tests for ammonia, chlorine and sulfur dioxide belong to G3 Science or G3 Chemistry. They are not required here.

2. Key Ideas

GasWhat to doPositive observation
carbon dioxidebubble the gas through limewaterlimewater turns milky
hydrogenapply a lighted splint to a small samplegas burns with a squeaky pop
oxygeninsert a glowing splintglowing splint relights

The test and the positive observation form one complete answer. Naming only the gas does not describe the test.

3. Detailed Explanations

Carbon dioxide

Bubble the gas through limewater. A milky suspension forms because insoluble calcium carbonate is produced:

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

Write “limewater turns milky”. “Turns white” is imprecise because the liquid becomes cloudy as a precipitate forms.

Hydrogen

Collect a small sample and apply a lighted splint. Hydrogen burns with a squeaky pop. The small-sample instruction is part of safe school-laboratory practice.

Oxygen

Insert a glowing splint into the gas. The splint relights because oxygen supports combustion. Do not use a lighted splint or describe a pop.

4. Common Mistakes

  • Swapping the lighted hydrogen splint with the glowing oxygen splint.
  • Writing “carbon dioxide puts out a flame”; this is not the prescribed identifying test.
  • Naming the gas but omitting the positive observation.
  • Treating a negative result as proof of a different gas.
  • Saying limewater “turns white” instead of “turns milky”.

5. Exam Tips

Write the complete evidence chain

Use procedure → observation → inference. For example: “Bubble the gas through limewater; the limewater turns milky, so carbon dioxide is present.”

Exam question 1: Choose the correct splintCore

An unknown gas must be tested for oxygen. State the splint condition and positive result.

Show Answer

Insert a glowing splint. It relights if oxygen is present.

6. Worked Examples

Modelled example 1

Identify Carbon Dioxide

Core

Problem

A gas is bubbled through limewater and the mixture becomes milky. State the observation and inference separately.
Study the worked solution

  1. Record what is seen

    Method

    State the colour or appearance change without naming the gas.

    Reason

    An observation must be directly detectable rather than inferred.

    Working

    Observation: limewater turns milky.

  2. Make the supported inference

    Method

    Identify carbon dioxide.

    Reason

    Turning limewater milky is the prescribed positive test for carbon dioxide.

    Working

    Inference: carbon dioxide is present.

Guided practice 2

Distinguish Hydrogen from Oxygen

About 5 min

Problem

Two unlabelled gas samples contain hydrogen and oxygen. Describe how to identify each.

Match condition and observation

Hydrogen test
Hydrogen positive result
Oxygen positive result

Hints

Hint 1: hydrogen
Hydrogen is combustible, so use a flame on a small sample.
Hint 2: oxygen
Oxygen supports combustion, so begin with a glowing splint.
View solution step by step

  1. Identify hydrogen

    Method

    Apply a lighted splint to a separate small sample.

    Reason

    Hydrogen gives the characteristic combustion result.

    Working

    A squeaky pop identifies hydrogen.

  2. Identify oxygen

    Method

    Insert a glowing splint into the other gas.

    Reason

    Oxygen supports combustion strongly enough to relight it.

    Working

    A glowing splint relights, identifying oxygen.

Common misconception 3

Correct an Incomplete Answer

Find and correct the mistake

Learner response

A learner writes, “Use a splint; it is oxygen.” Locate the first omission and improve the answer.

Diagnose the answer

First omission

View solution step by step

  1. Specify the procedure

    Method

    Insert a glowing splint into the gas.

    Reason

    A lighted splint is a different prescribed test, so “a splint” is ambiguous.

    Working

    Procedure: use a glowing splint.

  2. Restore observation and inference

    Method

    State that the splint relights, then identify oxygen.

    Reason

    The positive observation is the evidence supporting the inference.

    Working

    The glowing splint relights; therefore oxygen is present.

Examiner practice 4

Write a Markable Hydrogen Test

3 marks

Examination question

Describe the test for hydrogen and state the positive result. [3 marks]

Write the three-mark answer

View solution step by step

  1. State the sample and test

    1 mark

    Method

    Apply a lighted splint to a small sample of the gas.

    Reason

    Using a small sample limits the amount ignited.

    Working

    Bring a lighted splint to a small sample.

  2. State the observation

    1 mark

    Method

    Record the sound produced when the gas burns.

    Reason

    The characteristic positive result must be stated precisely.

    Working

    Observation: the gas burns with a squeaky pop.

  3. State the inference

    1 mark

    Method

    Identify hydrogen only after the positive observation.

    Reason

    The squeaky pop is the prescribed evidence for hydrogen.

    Working

    Inference: hydrogen is present.

Challenge 5

Interpret Three Gas-Test Results

Minimal support

New data

Samples A, B and C are known to be carbon dioxide, hydrogen and oxygen in some order. A turns limewater milky; B relights a glowing splint; C burns with a squeaky pop when tested with a lighted splint. Identify all three samples and justify each inference.

Interpret each positive result

Sample A
Sample B
Sample C

Hints

Hint 1: use-positive-results
Match each gas to a characteristic positive result, not to the absence of some other result.
Hint 2: three-pairs
Limewater–milky, glowing splint–relights, lighted splint–squeaky pop.
View solution step by step

  1. Use the limewater evidence

    Method

    Match A to the prescribed limewater result.

    Reason

    Carbon dioxide is the gas that turns limewater milky.

    Working

    A is carbon dioxide because it turns limewater milky.

  2. Use the glowing-splint evidence

    Method

    Match B to the prescribed glowing-splint result.

    Reason

    Oxygen supports combustion and relights a glowing splint.

    Working

    B is oxygen because it relights a glowing splint.

  3. Use the lighted-splint evidence

    Method

    Match C to the prescribed lighted-splint result.

    Reason

    Hydrogen burns with the characteristic squeaky pop.

    Working

    C is hydrogen because it burns with a squeaky pop.

7. Mind Stretchers

Mind stretcher 1: Explain the limit of a negative testExtension

A glowing splint does not relight. Why does this not prove that the gas is hydrogen?

Show Answer

Many gases do not relight a glowing splint. A positive hydrogen test still requires a small sample to burn with a squeaky pop when a lighted splint is applied.

Mind stretcher 2: Plan a three-gas sequenceExtension

Plan a safe sequence to identify three samples known to be carbon dioxide, hydrogen and oxygen.

Show Answer

Use separate portions. Test with limewater to identify carbon dioxide, apply a lighted splint to a small sample to identify hydrogen, and use a glowing splint to identify oxygen. Record each positive observation before the inference.

8. Quiz