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C10.3 Testing for Gases

GCSE Combined Science · AQA · Chapter C10: Chemical Analysis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: collecting gases, and the tests for hydrogen, oxygen, carbon dioxide and chlorine. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe ways of collecting a gas from a reaction
  • Describe the tests and positive results for hydrogen, oxygen, carbon dioxide and chlorine
  • Explain the limewater test and the squeaky pop using balanced equations
  • Identify gases made in reactions and in electrolysis from test results

Key words

  • Squeaky pop: The sound made when hydrogen mixed with air burns quickly at the mouth of a test tube.
  • Glowing splint: A wooden splint that has been lit and then blown out so that it is still glowing.
  • Limewater: Calcium hydroxide solution. It turns milky (cloudy) with carbon dioxide.
  • Litmus paper: An indicator paper. Damp litmus paper is bleached white by chlorine.
  • Bleached: Turned white because the colour has been removed.

Collecting a gas

Before testing a gas we usually collect it in a test tube. The method depends on the gas.

  • over water; Good for gases that are not very soluble in water, e.g. hydrogen, oxygen; Notes an upturned measuring cylinder also shows the volume
  • gas syringe; Good for any gas; Notes measures the volume accurately
  • upward delivery; Good for gases less dense than air, e.g. hydrogen; Notes gas goes up into an upside-down test tube
  • downward delivery; Good for gases denser than air, e.g. carbon dioxide, chlorine; Notes chlorine is toxic, so use a fume cupboard

The four gas tests

Each common gas has one quick test with a result that no other common gas gives. Learn the equipment, what you do with it and exactly what you see.

  • hydrogen, H₂; Test hold a burning (lighted) splint at the mouth of the test tube; Positive result burns rapidly with a squeaky pop
  • oxygen, O₂; Test put a glowing splint into the test tube; Positive result the splint relights
  • carbon dioxide, CO₂; Test bubble the gas through limewater (or shake it with limewater); Positive result limewater turns milky / cloudy
  • chlorine, Cl₂; Test hold damp litmus paper in the gas; Positive result paper is bleached white (blue litmus may turn red first)

Exam tip: Burning splint → hydrogen; glowing splint → oxygen. The litmus paper for chlorine must be damp. When asked how to identify a gas, always give both the test and the result.

What is happening in the tests?

Squeaky pop: hydrogen reacts quickly with oxygen in the air, making water: 2H₂(g) + O₂(g) → 2H₂O(l).

Glowing splint: oxygen supports burning, so the glowing wood bursts back into flame.

Limewater: carbon dioxide reacts with the calcium hydroxide to form insoluble calcium carbonate, which makes the solution milky: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).

Bleaching: chlorine dissolves in the water on damp paper and removes the dye.

Using the tests

  • hydrogen; A lab reaction that makes it a metal and a dilute acid, e.g. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
  • oxygen; A lab reaction that makes it hydrogen peroxide with a catalyst; electrolysis of a sulfate solution (anode)
  • carbon dioxide; A lab reaction that makes it a carbonate and an acid, e.g. MgCO₃(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l) + CO₂(g)
  • chlorine; A lab reaction that makes it electrolysis of sodium chloride solution – chlorine forms at the anode

Worked example: naming the gas

Sodium carbonate is added to dilute nitric acid. The gas made turns limewater cloudy.
The gas is carbon dioxide: Na₂CO₃(s) + 2HNO₃(aq) → 2NaNO₃(aq) + H₂O(l) + CO₂(g)

Answer: carbon dioxide

Worked example: metal + acid as an ionic equation (Higher tier)

Only the zinc and the hydrogen ions change. The sulfate ions are spectator ions.
Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)
Zinc atoms lose electrons, so zinc is oxidised. Hydrogen ions gain electrons, so they are reduced.

Answer: a redox reaction: Zn is oxidised, H⁺ is reduced

Common mistakes

  • Mixing up the splints: a burning splint is for hydrogen; a glowing splint is for oxygen.
  • Writing “limewater goes white” or “goes clear”. Write that it turns milky or cloudy.
  • Using dry litmus paper for chlorine. It must be damp.
  • Writing H for hydrogen gas or Cl for chlorine gas. The gases are made of molecules: H₂, O₂, Cl₂.

Revision questions

Try these, then check your answers below each one.

Question 1. Limewater is used to test for carbon dioxide. What is the chemical name of limewater?

Answer: calcium hydroxide

Working: Limewater is calcium hydroxide solution, Ca(OH)₂(aq).

Question 2. Zinc is added to dilute sulfuric acid. Which gas is produced? (Choose from: Hydrogen, Oxygen, Carbon dioxide, Sulfur dioxide.)

Answer: Hydrogen

Working: A metal and an acid make a salt and hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂.

Question 3. True or false? The test for chlorine uses dry litmus paper.

Answer: False

Working: False. The litmus paper must be damp. Chlorine bleaches damp litmus paper white.

Related lessons

Continue in the full course

This lesson is section C10.3 (AGCSC2703) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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