Acidity of Carbonyl group
Might as well make an answer of the comments.

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.

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.