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12.3 Testing for Gases

GCSE Chemistry · AQA · Chapter 12: Chemical Analysis

A free GCSE Chemistry lesson for AQA (8462) from OBA School: collecting gases, the four common gas tests, and how to test for water. 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 using a balanced equation
  • Describe how to test for water and how to show that water is pure
  • (Higher tier) calculate the volume of a gas at room temperature and pressure

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, Ca(OH)₂(aq). It turns milky with carbon dioxide.
  • Litmus paper: An indicator paper. Damp litmus paper is bleached white by chlorine.
  • Anhydrous copper(II) sulfate: A white solid with no water in it. It turns blue when water is added.

Collecting a gas

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

  • over water; Good for gases that are not very soluble in water, e.g. hydrogen, oxygen; Notes a measuring cylinder shows the volume collected
  • gas syringe; Good for any gas; Notes measures the volume accurately
  • upward delivery; Good for gases less dense than air, e.g. hydrogen; Notes tube points 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

Making a sample of each gas

To practise the tests, each gas can be made in the lab by a simple reaction.

  • hydrogen; A lab reaction that makes it a metal and a dilute acid: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g); Collect by over water or upward delivery
  • oxygen; A lab reaction that makes it hydrogen peroxide with a manganese(IV) oxide catalyst: 2H₂O₂(aq) → 2H₂O(l) + O₂(g); Collect by over water
  • carbon dioxide; A lab reaction that makes it a carbonate and a dilute acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g); Collect by downward delivery or gas syringe
  • chlorine; A lab reaction that makes it electrolysis of sodium chloride solution – chlorine forms at the positive electrode (anode); Collect by downward delivery, in a fume cupboard

Worked example: hydrogen as an ionic equation (Higher tier)

Only the magnesium and the hydrogen ions from the acid change. The chloride ions are spectator ions.
Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)
Magnesium atoms lose electrons, so magnesium is oxidised. Hydrogen ions gain electrons, so they are reduced.

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

The four gas tests

Each 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 lighted splint at the mouth of the test tube; Positive result burns 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 and turns white (blue litmus may turn red first)

The squeaky pop is the hydrogen reacting with oxygen in the air: 2H₂(g) + O₂(g) → 2H₂O(l)

Limewater turns milky because insoluble calcium carbonate forms: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Exam tip: Learn which splint goes with which gas: lighted 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 positive result.

Testing for water

  • Anhydrous copper(II) sulfate turns from white to blue when water is added: CuSO₄(s) + 5H₂O(l) ⇌ CuSO₄·5H₂O(s)
  • Blue cobalt chloride paper turns pink when water is added.
  • These tests show that water is present. They also work with salty or muddy water, so they do not show that the water is pure.
  • To show that water is pure, measure its boiling point: pure water boils at exactly 100 °C (and freezes at 0 °C) at normal pressure.

Using the tests

Worked example: naming the gas

Zinc is added to dilute hydrochloric acid. The gas made burns with a squeaky pop.
The gas is hydrogen: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Answer: hydrogen

Worked example: gas volume (Higher tier)

A carbonate reacts with acid and makes 0.060 mol of carbon dioxide. What volume is this at room temperature and pressure?
volume = moles × 24 dm³ = 0.060 × 24 = 1.44 dm³

Answer: 1.44 dm³ (= 1440 cm³)

Common mistakes

  • Mixing up the splints: a lighted 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.
  • Saying the copper sulfate test shows that water is pure. It only shows that water is present; use the boiling point (100 °C) for purity.
  • 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. Which is used to test for chlorine gas? (Choose from: Damp litmus paper, A glowing splint, Limewater, Anhydrous copper(II) sulfate.)

Answer: Damp litmus paper

Working: Chlorine bleaches damp litmus paper white.

Question 2. True or false? If anhydrous copper(II) sulfate turns blue when a liquid is added, the liquid must be pure water.

Answer: False

Working: The test shows only that water is present. To show the water is pure, check that it boils at exactly 100 °C.

Question 3. Limewater is a solution of which compound? Give its name.

Answer: calcium hydroxide

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

Related lessons

Continue in the full course

This lesson is section 12.3 (AGCCH1203) of our free GCSE Chemistry 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 Chemistry course.

 

 

 

 

 

 

 

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