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I m a high school chemistry learner. I have learnt that acids are substance that can liberate H+ on hydrolysis. For that they must be connected to an electronegative atom. In carbonyl ( mainly aldehyde and ketone) the H atom is not directly attached to Oxygen. So I may not consider it as an acid. But today while performing a chemistry practical when I added Blue litmus solution to aldehyde it turned red. This leads me to a confusion about whether they are acidic or neutral or What can I infer from the colour change of blue litmus.

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    $\begingroup$ How pure was the aldehyde? Aerial oxidation can easily produce carboxylic acid from aldehydes $\endgroup$
    – Waylander
    Feb 10 at 16:56
  • $\begingroup$ It gave Silver mirror test. The Formaldehyde may have been converted to Formic acid. Am i right? $\endgroup$
    – Pranav
    Feb 10 at 17:16
  • $\begingroup$ It would still give silver mirror if there was some acid present. Formaldehyde oxidises easily if not stored properly. It is good practice to purify your aldehyde before use in synthetic chemistry $\endgroup$
    – Waylander
    Feb 10 at 17:50
  • $\begingroup$ But then why didn't it showed reaction with Na2CO3 after that?? $\endgroup$
    – Pranav
    Feb 11 at 8:34
  • $\begingroup$ How did you react it with Na2CO3 and what did you observe? If it was only midly acidic there would not be enough CO2 generated for you to notice bubbles. $\endgroup$
    – Waylander
    Feb 11 at 8:49
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Might as well make an answer of the comments.

  1. Aldehydes are not only able to be oxidized to acids, they are quite reactive towards this oxidation. Exposure to air would be enough to trigger enough oxidation and acid formation to get litmus to react, especially if the litmus is moist. Aldehydes are not alone in this respect; for example iron(ii) salts can be oxidized by air too, especially if you try to store them as a solution or after breaking the original seal on the bottle.

  2. Moist litmus paper is a good qualitative test for acidity or basicity because it is highly sensitive to small amounts of acid or base. Reaction with sodium carbonate might not be as sensitive if you are looking for enough carbon dioxide to visibly form bubbles, especially as carbon dioxide has some solubility in most solvents.

Yes, you need to follow good practices inspecting, testing and if necessary, purifying sensitive chemicals if you want high precision or accuracy.

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