GCSE · Chemistry · AQA · Spec 8462

Test for chlorine

You know litmus as the paper that turns red or blue. In chlorine, damp litmus ends up neither: its colour disappears. That vanishing colour is the whole test.

Is it chlorine?

Seven results. Which ones show chlorine?

Each card is what a student saw after holding litmus paper in a gas. Make your call on each one first, then read why.

Still to sort

Shows chlorine (0)

Damp litmus paper, and its colour has gone: it's white.

Where the line is: A colour going is not the same as a colour changing. Only the paper turning white, with the colour removed, is the chlorine result.

Doesn't show chlorine (0)

A fair test with damp paper, but the paper still has colour at the end.

Where the line is: Red or blue at the end is an ordinary indicator colour. It isn't bleaching, so it doesn't show chlorine.

Not a proper test (0)

The paper wasn't damp, so this result can't tell you anything about chlorine.

Where the line is: This is not the same as 'no chlorine'. The chlorine test uses damp paper, so a dry-paper result doesn't count: do the test again with damp paper.

7 of 7 still to sort.

Watch out: The last card is the sneaky one. 'Nothing happened' only means 'no chlorine' when the paper was damp.

Say it properly

Which description is closest to yours?

A friend missed this lesson and asks you what chlorine does to damp litmus paper.

Which of these is closest to what you'd tell them?

It turns the paper red, because litmus goes red with acids.It turns the paper blue, like an alkali would.It turns the paper white, which is the colour litmus goes when a gas is neutral.It bleaches the paper: the colour is removed and the paper turns white.

How sure are you?

I'm sureI think soI'm guessing

It turns the paper red, because litmus goes red with acids.

This is a common misconception

You're right that litmus goes red with acids. That's litmus working as an indicator, swapping one colour for another. Chlorine does something different: it takes the colour away altogether, so the paper ends up white. If you write 'red', you've described an indicator, not the chlorine test.

It turns the paper blue, like an alkali would.

This is a common misconception

Blue is the other indicator colour of litmus, and it's still a swap: one colour in, another colour out. With chlorine, no colour is left at the end. The dye is destroyed, so the paper turns white, not blue.

It turns the paper white, which is the colour litmus goes when a gas is neutral.

This is a common misconception

You've got the white, but the reason is off. White isn't one of litmus's indicator colours, so it isn't telling you anything about acid or alkali. White is what's left when the dye has been destroyed. That's why the right word is 'bleached'.

It bleaches the paper: the colour is removed and the paper turns white.

This matches the evidence

Exactly. An indicator changes colour. Chlorine removes the colour. Say 'bleached', say 'white', and you've described the result correctly.

An indicator swaps one colour for another. Chlorine wipes the colour out. Write 'bleached', and say the paper turns white.

Why white?

Reason it through

Why is white paper the sign of chlorine, when red or blue isn't?

Link 1 of 3

First link · your turn

Here's the test: damp litmus paper goes into the gas. If the gas is chlorine, what happens to the paper?

Locked — reveal the link above first

Your two-question check

Reading any litmus result

Take any result you're given. Ask the first question, follow the branch that's true, then ask the second.

Was the paper damp? → Has the colour gone?

3 possible verdicts.

On Litmus paper has been held in a gas. 2 branches to choose from.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The colour doesn't just change. It disappears.

What you need to know

The test for chlorine uses damp litmus paper.
When damp litmus paper is put into chlorine gas, the paper is bleached: its colour is removed and it turns white.
Bleaching is not an indicator colour change. A paper that ends up red or blue has not been bleached, so it doesn't show chlorine.
A test with dry litmus paper isn't the chlorine test, so 'no change' on dry paper doesn't mean 'no chlorine'.

The big picture

Chlorine is identified with damp litmus paper. In chlorine gas the damp paper is bleached: its colour is removed and it turns white. That's different from an indicator colour change such as red or blue. The paper has to be damp, because that's how the test is done, so a result from dry paper isn't the chlorine test and can't tell you whether chlorine is there.

Key points

1Damp litmus paper plus chlorine gives bleached, white paper.

2Colour gone, not colour changed: the result is white, never red or blue.

3Damp paper, every time. Dry paper is not the test.

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Worked example

Problem

A student writes this in their notes: 'Test for chlorine: hold litmus paper in the gas. If it turns red, the gas is chlorine.' Find the two things wrong with it, and write a correct version.

⚠ Watch out

Writing that the litmus paper 'turns red'. Red is an indicator colour. The chlorine result is that damp litmus paper is bleached and turns white. The other slip is leaving out the word 'damp'.

🧠

Memory hook

Chlorine doesn't paint the paper, it wipes it clean. Damp paper in, white paper out.

✓

Check yourself

Without looking back: name the two things that must be true for a litmus test to show chlorine. Then say what you'd do if the paper had been dry.

Flashcards

(6)

What does chlorine do to damp litmus paper?

It bleaches it. The colour is removed and the paper turns white.

A method says: 'Hold litmus paper in the gas.' What's missing from this chlorine test?

The word 'damp'. The chlorine test uses damp litmus paper, so dry paper isn't the test.

Damp blue litmus paper turns red and stays red in a gas. Does that show chlorine?

No. Red is an ordinary indicator colour change (it shows an acid). The chlorine result is the colour disappearing completely.

Dry litmus paper stays the same colour in a gas. Can you say there's no chlorine?

No. The chlorine test uses damp paper, so this wasn't the test done properly. Repeat it with damp paper.

How is bleaching different from what an indicator does?

An indicator swaps one colour for another. Bleaching destroys the dye, so no colour is left.

Damp red litmus paper turns white in a gas. What is the gas?

Chlorine. Red or blue litmus, the chlorine result is the same: the paper is bleached white.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Seven results. Which ones show chlorine?

Which description is closest to yours?

Reading any litmus result

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