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The test for chlorine gas is to hold damp litmus paper near the gas: chlorine bleaches it white. Free GCSE Chemistry notes and 10 FAQs.
Testing for chlorine as a Group 7 halogen is part of The periodic table in GCSE Edexcel Higher Chemistry.
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The test for chlorine gas is to hold damp litmus paper near the gas: chlorine bleaches it white. Free GCSE Chemistry notes and 10 FAQs.
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Use this lesson to stay inside The periodic table while connecting the detail back to the wider Topic 6: Groups in the periodic table topic.
Read the core explanations from Testing for chlorine as a Group 7 halogen 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 positive result for chlorine gas is that damp litmus paper is bleached and turns white. Saying the paper turns red or blue is wrong, because bleaching removes the colour completely.
Hypochlorous acid, HClO, is the bleaching agent formed when chlorine reacts with water. It is the same chemical used in swimming pools and household bleach.
Chlorine gas is tested for by holding damp litmus paper near the gas in a fume cupboard, keeping the face away and not breathing it in. Chlorine is toxic and irritates the lungs, and a positive test bleaches the paper white.
The test for chlorine gas is to dampen a piece of red or blue litmus paper with distilled water and hold it near the gas. If chlorine is present, the paper is bleached and turns white within seconds.
Bleaching is the removal of colour from a substance by breaking down the molecules responsible for colour. Chlorine bleaches damp litmus paper white, which is how chlorine gas is identified.
Litmus paper must be damp because chlorine only forms a bleaching agent when water is present. Chlorine reacts with water to form hypochlorous acid (HClO), so dry litmus paper gives no reaction.
Chlorine bleaches litmus paper because it reacts with water to form hypochlorous acid: Cl₂ + H₂O → HCl + HClO. The hypochlorous acid breaks down the organic molecules that give litmus its colour, so the bleaching is permanent.
In the electrolysis of sodium chloride solution, chlorine forms at the positive electrode (anode): 2Cl⁻ → Cl₂ + 2e⁻. Chlorine should be tested in a fume cupboard because it is toxic and irritates the respiratory system.
Bleaching by chlorine is permanent: chlorine destroys the colour molecules completely, so the colour cannot come back. This makes the chlorine test different from other gas tests, whose colour changes could in theory be reversed.
The colour of the litmus paper does not matter for the chlorine test: red or blue litmus will both be bleached white. What matters is that the paper is damp.