AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemical analysis › Identification of common gases
Practise Test for chlorine. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How to show that a gas is chlorine: damp litmus paper is bleached white. Questions ask for the test and result, often linked to the electrolysis of chloride solutions, to safety with this toxic gas, or to all four gas tests together.
Key facts
Test: put damp litmus paper into the gas
Result: the litmus paper is bleached (turns white)
The litmus paper must be damp
Chlorine is toxic: use a fume cupboard
Electrolysis of a chloride solution: chlorine at the positive electrode (anode)
If the gas is chlorine, the litmus paper is bleached and turns white.
Chlorine bleaches damp litmus paper white.
Blue litmus may turn red for a moment first, because chlorine forms an acidic solution in water. The result to give is 'bleached' or 'turns white'.
The paper must be damp: chlorine has to dissolve in the water on the paper to bleach it.
About chlorine
Chlorine is a pale yellow-green gas with a sharp smell.
Chlorine is toxic: use only small amounts and do the test in a fume cupboard.
Chlorine forms at the positive electrode (anode) when a solution containing chloride ions, such as sodium chloride solution, is electrolysed. Hydrogen forms at the negative electrode.
The four gas tests together
Hydrogen: burning splint at the open end of the tube → pop.
Oxygen: glowing splint put into the tube → relights.
Carbon dioxide: bubble through limewater → limewater turns milky.
Chlorine: damp litmus paper → bleached white.
How to answer each type of question
Describe the test for chlorine and give the result
2 marksGrade 3
Test (1 mark): damp litmus paper put into the gas.
Result (1 mark): the paper is bleached / turns white.
Example. Sodium chloride solution is electrolysed. A gas forms at the positive electrode. Describe a test to show that this gas is chlorine. Give the result of the test.
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Put damp litmus paper into the gas (1). The litmus paper is bleached / turns white (1).
Give a hazard and a safety precaution
2 marksGrade 4
Hazard: chlorine is toxic.
Precaution: do the test in a fume cupboard / use only a small amount.
Example. Give one hazard of chlorine gas and one precaution a student should take when testing for it.
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Chlorine is toxic (1). Carry out the test in a fume cupboard / use only a small amount of the gas (1).
Don’t lose marks
Forgetting to say the litmus paper is damp, or using dry paper.
Giving 'turns red' as the result. Blue litmus may go red first, but the result is bleached / white.
Forgetting the safety point: chlorine is toxic, so use a fume cupboard.
Mixing up the electrodes: in the electrolysis of sodium chloride solution, chlorine forms at the positive electrode and hydrogen at the negative electrode.
More tips
Memory tricks
Chlorine bleaches: the colour goes out of damp litmus paper.
Damp for chlorine: think of the chlorine smell of a (wet) swimming pool.
The four results in order H2, O2, CO2, Cl2: 'Pop, Relight, Milky, Bleach'.
Exam technique
Use the spec words: 'damp litmus paper' and 'bleached' or 'turns white'.
In electrolysis questions, name the gas at each electrode and give a test for each.
Learn all four gas tests together; a whole question often tests them side by side.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Describe the test for chlorinePut damp litmus paper into the gas.
Grade 3
State the positive result for chlorineThe litmus paper is bleached and turns white.
Grade 4
Give a safety precaution for chlorineChlorine is toxic, so use small amounts in a fume cupboard.
Grade 5
Name a reaction that produces chlorineElectrolysis of a chloride solution, such as sodium chloride solution: chlorine forms at the positive electrode.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Sodium chlorate(I) has the formula NaClO. Balance the equation for the reaction between sodium chlorate(I) and hydrochloric acid. NaClO + ....HCl → NaCl + Cl2 + H2O
NaClO + 2HCl → NaCl + Cl2 + H2O
A student wants to find out whether a gas is chlorine. Name the type of paper used in the test for chlorine.
(damp) litmus paper
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
(a) Which test result shows that a gas is chlorine? Tick (✓) one box.[1]
Damp litmus paper is bleached
Damp red litmus paper turns blue
A glowing splint relights
Limewater turns milky
(b) Why must the litmus paper be damp?[1]
(c) Give one safety precaution a student should take when testing for chlorine.[1]
Show the answer and mark scheme
(a)Answer: Damp litmus paper is bleached
(b)
so that the chlorine can dissolve in / react with the water (to bleach the paper)
(c)
use a fume cupboard / well-ventilated room (because chlorine is toxic)
Question 2Medium3 marks
A student electrolysed sodium chloride solution. A pale green gas was produced at the positive electrode.
(a) Describe the test to show that the gas is chlorine. Give the result.[2]
(b) Carbon dioxide is also an acidic gas. Suggest why carbon dioxide does not give the same result as chlorine in this test.[1]
Show the answer and mark scheme
(a)
hold damp litmus paper in the gas
the paper is bleached / turns white
(b)
carbon dioxide does not bleach the (litmus) paper / carbon dioxide would only turn (blue) litmus paper red