I am wondering what could cause my litmus paper to first react to the alkaline. turning blue, and then changing to an acidic red color?

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    $\begingroup$ Was the paper being held in solution during the color change, or was it whetted then turned red while sitting in the air? $\endgroup$ – airhuff May 30 '17 at 18:23

Litmus paper contains a pH indicator, which is a compound that changes color in the presence of acid or base. Natural litmus contains numerous pH-sensitive dyes, but most of them are based on the 7-hydroxyphenoxazone core. In the presence of acid, the imine nitrogen is protonated; in base, the phenolic oxygen is deprotonated. These reactions are reversible, so the color will change when the pH is changed.

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  • $\begingroup$ While you successfully explained how an pH indicator works, the question at hand remains unclear because it is missing details. I do, however, expect that the OP knows that the pH has changed. I guess the OP is concerned with why. $\endgroup$ – Martin - マーチン Jun 1 '17 at 10:20
  • $\begingroup$ @Martin-γƒžγƒΌγƒγƒ³ I'm uncertain of what you would expect to be done in this situation. The two options, as I see it, are to try to answer the question the best I can given the inherent vagueness of the question, or to downvote the question without attempting to provide any answer in the hopes of the OP editing the original question for clarity. The OP refers specifically to the colour change of litmus as opposed to why the pH has changed, and I would argue that this therefore answers the question. $\endgroup$ – JSK Jun 1 '17 at 10:33
  • $\begingroup$ I would expect the question to be closed as unclear, additionally I think it warrants a down-vote. I don't think your answer is bad, but it won't be helpful in the long term of this site. Questions as vague as this clutter the site and prevent other questions to be effectively found. Your answer prevents the question from being deleted, which means it might have to be cleaned up manually in the future. $\endgroup$ – Martin - マーチン Jun 1 '17 at 10:38

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