Tests for gases: hydrogen, oxygen, carbon dioxide and chlorine

The test for hydrogen is to hold a burning splint at the open end of a test tube of the gas. If it is hydrogen, it burns rapidly with a squeaky pop. The other GCSE gas tests are: a glowing splint relights in oxygen, carbon dioxide turns limewater milky, and chlorine bleaches damp litmus paper white.

Tests for gases: summary

These are the four gas tests in AQA GCSE Chemistry (4.8.2, Identification of common gases). Learn the test and the result as a pair: a mark scheme usually wants both.

Test for hydrogen: the squeaky pop test

1. Collect the gas in a test tube and keep it closed with your thumb or a bung.
2. Light a wooden splint so it is burning.
3. Take the bung off and hold the burning splint at the mouth of the tube.
4. A squeaky pop means the gas is hydrogen.

The pop is a tiny explosion: hydrogen mixed with the oxygen in the air burns very quickly, making water.

Hydrogen is given off when a reactive metal such as magnesium or zinc reacts with a dilute acid, and at the negative electrode in the electrolysis of many solutions. For example:

Test for oxygen: the glowing splint

1. Light a splint, then blow it out so that it is still glowing at the tip.
2. Put the glowing splint into the test tube of gas.
3. If the splint relights and bursts into flame, the gas is oxygen.

A glowing splint keeps smouldering in air but doesn't catch fire, because air is only about a fifth oxygen. In pure oxygen it burns much faster, so it relights. Oxygen is made, for example, when hydrogen peroxide decomposes with a manganese(IV) oxide catalyst, and at the positive electrode when some solutions are electrolysed.

Test for carbon dioxide: limewater

Limewater is a solution of calcium hydroxide in water. Bubble the gas through it, or shake the gas with it. If the gas is carbon dioxide, the limewater turns milky (cloudy), because a fine white precipitate of calcium carbonate forms:

Carbon dioxide is made when a carbonate reacts with an acid, when fuels burn, and in respiration. For example, the test for a carbonate is to add a dilute acid and test the gas given off with limewater.

Test for chlorine: damp litmus paper

Hold a piece of damp litmus paper in the gas. If it is chlorine, the paper is bleached and turns white. Blue litmus may turn red for a moment first, because chlorine forms acids when it dissolves in the water on the paper. The paper must be damp: dry litmus paper doesn't work.

Chlorine is toxic, so this test should be done in a fume cupboard. Chlorine is made at the positive electrode in the electrolysis of sodium chloride solution.

Check your understanding.

Worked examples: identifying a gas

Example 1: a metal and an acid

Zinc is added to dilute sulfuric acid and a colourless gas fizzes off. How could you show it is hydrogen? Collect the gas in a test tube and hold a burning splint at the open end. A squeaky pop shows it is hydrogen. (Zn + H₂SO₄ → ZnSO₄ + H₂)

Example 2: a carbonate and an acid

Marble chips (calcium carbonate) react with hydrochloric acid. Name the gas and give the test. The gas is carbon dioxide: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Bubble it through limewater, which turns milky.

Example 3: electrolysis of sodium chloride solution

Two gases are collected. How can you tell which is which? Test each with damp litmus paper and a burning splint. The gas from the positive electrode bleaches damp litmus paper: chlorine. The gas from the negative electrode burns with a squeaky pop: hydrogen.

Common mistakes

• Using a glowing splint for hydrogen or a lit splint for oxygen.
• Saying the splint "goes out" for hydrogen. The result is a squeaky pop.
• Saying limewater turns "white" or "clear". It turns milky or cloudy.
• Leaving out "damp" in the chlorine test, or saying the paper turns red. The result is bleached white.
• Giving only the result without the test, or the test without the result.

Practice questions

1. Describe the test for oxygen and give the result.
2. Write a word equation for the reaction that turns limewater milky.
3. Why must the litmus paper be damp when testing for chlorine?
4. A student adds calcium to water and collects a gas that pops with a burning splint. Name the gas.

Answers: 1) Put a glowing splint into the gas; it relights. 2) Calcium hydroxide + carbon dioxide → calcium carbonate + water. 3) Chlorine needs to dissolve in water to bleach the paper. 4) Hydrogen.

Practise with a short timed paper

Gas tests and chemical analysis come up on both tiers of GCSE Chemistry. The daily GCSE Chemistry Higher and GCSE Chemistry Foundation papers are short timed papers of 8 questions, with a new one every day.

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