Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
We used benzaldehyde for a demonstration of the Cannizzaro reaction in school, but the liquid we used appeared visibly yellow, as opposed to the colourless liquid described by Wikipedia.
A few reasons I could think of include:
I haven't found much discussion about this, nor do I have any candidate compounds for if my guesses above turn out to be right.
Tl;DR: All three factors are responsible for coloring of benzaldehyde (and pretty much any organics)
For the first two factors, I would mention the paper1 linked by @Andrew. Benzaldehyde is prone to atmospheric oxidation; the reaction is accelerated in presence of light to form primarily benzoic acid mixed other aromatics and polymerized aromatics causing darkening.
For the impurity aspect, let me quote the following from a paper2 discussing about benzaldehyde purification process:
[...] It is clear that traces of impurities, namely metals, influence markedly the benzaldehyde activity. The active impurities present in pure benzaldehyde or in that of anal, grade become visible after distillation leaving the distillation residue brown to yellow coloured. The criterion for the good quality of benzaldehyde is the colorless distillation residue.
My previous discussions:
References
