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The test for chlorine is to hold damp litmus paper near the gas: chlorine bleaches the paper and turns it white. Free notes and 10 questions answered.
Testing for Chlorine Gas is part of Identification of common gases in GCSE AQA Higher Chemistry.
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The test for chlorine is to hold damp litmus paper near the gas: chlorine bleaches the paper and turns it white. Free notes and 10 questions answered.
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Use this lesson to stay inside Identification of common gases while connecting the detail back to the wider Topic 8: Chemical analysis topic.
Read the core explanations from Testing for Chlorine Gas before testing yourself from memory.
Chlorine gas is colourless but deadly. You can't see it, but you need to identify it quickly in the lab. Whether it's produced during electrolysis or released from bleach reactions, knowing how to test for chlorine could save your life.
The test is beautifully simple. Chlorine has one unique property that no other common gas shares - it bleaches things permanently. This makes identification foolproof.
Here's the complete method. Take a piece of litmus paper - red or blue doesn't matter. Make it damp with distilled water. Hold it near the gas you're testing. If it's chlorine, the paper will lose all its colour and turn white.
The key word is 'damp'. Dry litmus paper won't work because chlorine needs water to form the bleaching agent. Think of it like activating a chemical reaction.
Let's check you've got the basics before we dig deeper into why this works.
Chlorine is a powerful oxidising agent. When it meets water on the damp litmus paper, it forms hypochlorous acid (HClO). This acid breaks down the organic molecules that give litmus its colour.
The reaction is: $Cl_2 + H_2O \rightarrow HCl + HClO$. The hypochlorous acid (HClO) is what does the bleaching. It's the same chemical used in swimming pools and household bleach.
This bleaching is permanent. Unlike indicators that change colour reversibly, chlorine destroys the colour molecules completely. You can't get the colour back.
Many students confuse bleaching with pH changes. Let's test your understanding.
Worked example: Testing for Chlorine Gas
You suspect a gas sample contains chlorine. Describe how to test for it safely.
Answer: Positive test: damp litmus paper turns white due to bleaching
Safety note: Always test gases in a fume cupboard. Chlorine irritates your respiratory system and can cause serious damage in high concentrations.
You'll encounter chlorine gas in several GCSE contexts. During electrolysis of sodium chloride solution, chlorine forms at the positive electrode (anode). The reaction is: $2Cl^- \rightarrow Cl_2 + 2e^-$.
Chlorine also forms when you mix bleach with acids - this is extremely dangerous and should never be done deliberately. It's why you must never mix cleaning products.
Can you identify where chlorine comes from in these scenarios?
Each gas has its own specific test. Hydrogen pops with a lighted splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. But only chlorine bleaches damp litmus paper white.
The chlorine test is unique because it's the only one that destroys the indicator permanently. All other gas tests involve colour changes that could theoretically be reversed.
Here's where students often get confused between different gas tests.
Examiners love to test this because students make predictable mistakes. The most common error is forgetting that the litmus paper must be damp. Dry paper gives no reaction.
Another trap is confusing bleaching with pH changes. Some students write 'turns red' or 'turns blue' instead of 'turns white'. Remember: bleaching removes colour completely.
Let's finish with a classic exam question that catches many students out.
Exam precision note: write that damp litmus paper is used and that chlorine bleaches it white. If you explain why, state that chlorine forms a bleaching agent only when water is present, which is why the paper must be damp.
Chlorine only bleaches litmus paper when water is present, which is why the paper must be damp. In exams, saying only that the paper changes colour is not enough; the mark-winning detail is that it is bleached white.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The test for chlorine is to hold damp litmus paper near the gas: chlorine bleaches the paper and turns it white. Either red or blue litmus paper can be used.
Bleaching is the removal of colour from a substance by breaking down the molecules responsible for the colour. Chlorine bleaches damp litmus paper white permanently, unlike indicators that change colour reversibly.
Chlorine reacts with water to form HCl and hypochlorous acid: Cl₂ + H₂O → HCl + HClO. The hypochlorous acid (HClO) is what does the bleaching, and it is the same chemical used in swimming pools and household bleach.
Hydrogen pops with a lighted splint, oxygen relights a glowing splint, carbon dioxide turns limewater milky, and chlorine bleaches damp litmus paper white. The chlorine test is the only one that destroys the indicator permanently.
Chlorine bleaches litmus paper because, with water, it forms hypochlorous acid (HClO), which breaks down the molecules that give litmus its colour. The bleaching is permanent, unlike an indicator colour change.
The litmus paper must be damp because chlorine needs water to form the bleaching agent, hypochlorous acid: Cl₂ + H₂O → HCl + HClO. Dry litmus paper gives no reaction.
Chlorine gas forms at the anode during electrolysis of sodium chloride solution: 2Cl⁻ → Cl₂ + 2e⁻. It also forms when bleach is mixed with acids, which is extremely dangerous, so cleaning products must never be mixed.
It doesn't matter whether red or blue litmus paper is used in the chlorine test; both are bleached white. What matters is that the litmus paper is damp, because chlorine needs water to form the bleaching agent.
Chlorine gas is toxic: it irritates the respiratory system and can cause serious damage in high concentrations. Gases should be tested in a fume cupboard, holding the damp litmus paper near the gas without breathing it in.
Bleach should never be mixed with acids because chlorine gas forms, which is toxic. This is why cleaning products must never be mixed, and why gas tests should be done in a fume cupboard.