Test for chlorine gas: why does damp litmus paper become bleached?

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Alpha-Omega
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I know that it turns blue and red when it loses/keeps a hydrogen atom from the litmus molecule, but how does it become bleached (like white)?

Is this due to the chlorine oxidising the litmus paper?

Also, why used damped litmus paper? What role do the water molecules play in here?
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nevetstreblig
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Cl2 (g) + H2O (l) --> HClO (aq) + HCl (aq)

HCl bleaches the litmus blue, however HClO bleaches the litmus white
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charco
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(Original post by nevetstreblig)
Cl2 (g) + H2O (l) --> HClO (aq) + HCl (aq)

HCl bleaches the litmus blue, however HClO bleaches the litmus white
HCl turns the litmus red - it doesn't bleach it red. The word 'bleach' is used to show that the colour is being lost, i.e. it is turning white.

HOCl is the bleach in the reaction.

The reason why it bleaches organic matter probably is due to loss of conjugation because of addition reactions occuring when the H-OCl meets a double bond.

This destroys the extended conjugation (chromophore) and the colour is lost.
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charco
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(Original post by Alpha-Omega)

Also, why used damped litmus paper? What role do the water molecules play in here?
Gases do not react with solids. It is standard practise to dampen the indicator paper when testing for all gases.

The water allows a small amount of the gas to dissolve and come into contact with the dyes in the indicator paper.
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Bob _ross
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Thanks
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Dave_Rado
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Hi harco

Why doesn't Fluorine also bleach damp litmus paper? Why only chlorine?

Dave
Last edited by Dave_Rado; 3 weeks ago
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Davies Chemistry
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(Original post by Dave_Rado)
Hi harco

Why doesn't Fluorine also bleach damp litmus paper? Why only chlorine?

Dave
Fluorine will bleach it too- but almost nobody works with fluorine- it is so reactive it is hard to obtain in the form of the element. No chemical reactions ever result in the liberation of F2 gas. The only way to make it is by electrolysis.
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scimus63
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The reactions of HF are generally different from the other hydrogen halides. Whereas HCl, HBr, HI all dissolve to form strong acids , HF does not, at least I think in dilute solutions. I think some of the differences are due to the strong hydrogen bonding that occurs with HF.

HF gas is very toxic and extremely corrosive, even small amounts are fatal, best avoided!
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Dave_Rado
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(Original post by Davies Chemistry)
Fluorine will bleach it too- but almost nobody works with fluorine- it is so reactive it is hard to obtain in the form of the element. No chemical reactions ever result in the liberation of F2 gas. The only way to make it is by electrolysis.
Many thanks Davies Chemistry - but what about Bromine gas? It's less reactive than chlorine but still capable of addition reactions with a double carbon bond.
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Davies Chemistry
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(Original post by Dave_Rado)
Many thanks Davies Chemistry - but what about Bromine gas? It's less reactive than chlorine but still capable of addition reactions with a double carbon bond.
I think bromine vapour could probably bleach litmus too. The bleaching action is down to the oxidising nature of Cl2 i.e. it is good at removing electrons. Bromine is also oxidising, but not as strong as chlorine.
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Dave_Rado
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Thanks again Davies Chemistry - you seem to be right - I've just found this here: "One test for Bromine is based upon its oxidizing characterisitcs. Bromine is a bleach. Used on moist litmus paper, it reacts making the latter go completely colourless even though this reaction is rather slow."

But it still seems odd to me that the litmus bleaching test is almost always cited as a test specifically for chlorine gas and not for bromine gas. Any thoughts on why bromine gas is almost never mentioned in this context?

Dave
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