Identification of Gases

This lesson covers the tests for common gases as required by the AQA GCSE Chemistry specification (8.1.2). You need to know how to test for and identify hydrogen, oxygen, carbon dioxide, and chlorine. These gas tests are fundamental to chemical analysis and appear regularly in exams — both as standalone questions and within the context of reactions that produce gases.

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Why Do We Test for Gases?

Gas tests allow chemists to:

• Identify unknown gases produced in chemical reactions.
• Confirm that a predicted reaction has taken place.
• Monitor industrial processes.
• Ensure safety — some gases (e.g., chlorine) are toxic.

You must be able to describe the test AND the positive result for each gas.

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Test for Hydrogen (H\u2082)

Feature	Detail
Test	Hold a burning splint near the mouth of the test tube containing the gas
Positive result	The hydrogen burns with a squeaky pop
Why it works	Hydrogen is flammable and reacts explosively with oxygen in the air: 2H\u2082 + O\u2082 \u2192 2H\u2082O

When Hydrogen Is Produced

Hydrogen gas is commonly produced when:

• A metal reacts with an acid (e.g., Mg + 2HCl \u2192 MgCl\u2082 + H\u2082)
• A reactive metal reacts with water (e.g., 2Na + 2H\u2082O \u2192 2NaOH + H\u2082)

Exam Tip: The "squeaky pop" test for hydrogen is simple but must be described precisely. Say "a burning splint is held at the mouth of the test tube" — do NOT say "a lit splint is put into the gas." The splint must be at the opening, not inside the tube. Also, the word "pop" alone is not enough — you must say "squeaky pop."

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Test for Oxygen (O\u2082)

Feature	Detail
Test	Insert a glowing splint into the gas
Positive result	The glowing splint relights
Why it works	Oxygen supports combustion; it allows the glowing ember to burst back into flame

When Oxygen Is Produced

Oxygen gas is commonly produced when:

• Hydrogen peroxide decomposes (often with a manganese dioxide catalyst): 2H\u2082O\u2082 \u2192 2H\u2082O + O\u2082
• Metal oxides or peroxides are heated
• During photosynthesis in plants

Important Distinction: Burning vs Glowing Splint

Splint Type	Used to Test For	Positive Result
Burning splint (with flame)	Hydrogen	Squeaky pop
Glowing splint (no flame, just ember)	Oxygen	Relights

Exam Tip: Do NOT confuse the hydrogen test with the oxygen test. Hydrogen uses a burning splint (squeaky pop). Oxygen uses a glowing splint (relights). A common mistake is saying "a lit splint relights" — a lit splint is already burning, so it cannot "relight." The splint must be glowing (the flame has been blown out, leaving a glowing ember).

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Test for Carbon Dioxide (CO\u2082)

Feature	Detail
Test	Bubble the gas through limewater (calcium hydroxide solution, Ca(OH)\u2082)
Positive result	The limewater turns milky (cloudy / chalky white)
Why it works	Carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate: CO\u2082 + Ca(OH)\u2082 \u2192 CaCO\u2083 + H\u2082O

When Carbon Dioxide Is Produced

Carbon dioxide is commonly produced when:

• A carbonate reacts with an acid (e.g., CaCO\u2083 + 2HCl \u2192 CaCl\u2082 + H\u2082O + CO\u2082)
• During combustion of hydrocarbons and organic compounds
• During respiration in living organisms
• When carbonates or hydrogen carbonates are heated (thermal decomposition)

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Test for Chlorine (Cl\u2082)

Feature	Detail
Test	Hold a piece of damp litmus paper (or damp universal indicator paper) in the gas
Positive result	The litmus paper is bleached white (decolourised)
Why it works	Chlorine dissolves in water to form a mixture of hydrochloric acid and hypochlorous acid (a bleaching agent): Cl\u2082 + H\u2082O \u2192 HCl + HOCl

Important Notes About the Chlorine Test

• The litmus paper must be damp — dry litmus paper will not work because chlorine needs water to form the bleaching agent.
• The paper may initially turn red (because of the acid formed) before being bleached white.
• Chlorine is a toxic green-yellow gas with a pungent smell — handle with care in a fume cupboard.

Exam Tip: For the chlorine test, always say "damp litmus paper is bleached white." If you just say "turns white," you may lose the mark because "bleached" is the key term. Also note that the paper turns red first (due to acid) then is bleached — if you are asked to describe the full observation, mention both colour changes.

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Summary of Gas Tests

graph LR
    A["Gas to Identify"] --> B["Hydrogen"]
    A --> C["Oxygen"]
    A --> D["Carbon Dioxide"]
    A --> E["Chlorine"]

    B --> B1["Burning splint"]
    B1 --> B2["Squeaky pop"]

    C --> C1["Glowing splint"]
    C1 --> C2["Relights"]

    D --> D1["Limewater"]
    D1 --> D2["Turns milky / cloudy"]

    E --> E1["Damp litmus paper"]
    E1 --> E2["Bleached white"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#c0392b,color:#fff

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Quick Reference Table

Gas	Formula	Test	Positive Result
Hydrogen	H\u2082	Burning splint	Squeaky pop
Oxygen	O\u2082	Glowing splint	Relights
Carbon dioxide	CO\u2082	Limewater	Turns milky / cloudy
Chlorine	Cl\u2082	Damp litmus paper	Bleached white

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Applying Gas Tests in Context

In the exam, you may be asked to identify a gas produced in a reaction. For example:

Question: A student adds hydrochloric acid to calcium carbonate. Describe how the student could test the gas produced and state the expected result.

Answer: The student should bubble the gas through limewater. If the gas is carbon dioxide, the limewater will turn milky (cloudy). This confirms that carbon dioxide is produced because calcium carbonate reacts with hydrochloric acid: CaCO\u2083 + 2HCl \u2192 CaCl\u2082 + H\u2082O + CO\u2082.

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Safety Considerations