Testing for Gases
Revise Testing for Gases for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Learn the four gas tests: hydrogen (squeaky pop), oxygen (relights a glowing splint), carbon dioxide (limewater turns milky) and chlorine (bleaches litmus).
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Four gas tests must be known precisely, including both the method and the observation.
Hydrogen: hold a lighted splint at the mouth of the tube. It burns rapidly with a squeaky pop. Oxygen: insert a glowing splint. It relights.
Carbon dioxide: bubble the gas through limewater. It turns from colourless to milky, or cloudy. Chlorine: hold damp litmus paper in the gas. It is bleached white. Confusing the lighted and glowing splint tests is the most common error, so learning which goes with which matters.
Revision notes
Hydrogen and oxygen
Hydrogen: a lighted splint burns rapidly with a squeaky pop.
Oxygen: a glowing splint relights. Note the difference — hydrogen uses a lighted splint, oxygen uses a glowing one. This distinction is examined regularly.
Carbon dioxide
Bubble the gas through limewater, which is calcium hydroxide solution.
If carbon dioxide is present the limewater turns from colourless to milky or cloudy. This is caused by insoluble calcium carbonate forming.
Chlorine
Hold damp litmus paper in the gas.
Chlorine bleaches the litmus paper white. The paper must be damp for the test to work, which is a detail worth including.
Key points
- Hydrogen: lighted splint gives a squeaky pop.
- Oxygen: glowing splint relights.
- Carbon dioxide: limewater turns milky.
- Chlorine: damp litmus paper is bleached white.
- Limewater is calcium hydroxide solution.
- The litmus paper must be damp for the chlorine test.
Worked examples
Example 1
Describe the test for oxygen and the positive result. [2 marks]
Model answer
Insert a glowing splint into the test tube (1 mark). If oxygen is present the splint relights (1 mark).
Example 2
Describe the test for carbon dioxide and the positive result. [2 marks]
Model answer
Bubble the gas through limewater (1 mark). If carbon dioxide is present the limewater turns from colourless to milky or cloudy (1 mark).
Example 3
A student uses a lighted splint to test for oxygen. Explain why this is incorrect. [2 marks]
Model answer
A lighted splint is the test for hydrogen, which gives a squeaky pop (1 mark). Oxygen requires a glowing splint, which relights in its presence (1 mark).
Common mistakes
Confusing lighted and glowing splints.
Lighted splint is for hydrogen; glowing splint is for oxygen.
Saying limewater turns blue.
It turns milky or cloudy, from colourless.
Using dry litmus paper for chlorine.
The paper must be damp for the bleaching to occur.
Giving only the method without the observation.
Both are needed — the marks are usually split between them.
Exam tips
- Learn all four tests as method plus observation.
- Keep lighted and glowing splints clearly separate.
- Say damp litmus paper for the chlorine test.
- Describe limewater going from colourless to milky.
Key terms
- Limewater
- Calcium hydroxide solution, used to test for carbon dioxide.
- Glowing splint
- A splint used to test for oxygen.
- Lighted splint
- A splint used to test for hydrogen.
- Bleached
- Turned white, as damp litmus is by chlorine.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.