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While testing for formaldehyde (an aldehyde), I added Fehling's solution to the sample and let the test tube simmer in a water bath for some time. I returned ~15 minutes later to see a black/dark grey precipitate awaiting me, which looked very metallic (Almost like Tollen's test, but darker)

I try again and this time heat the tube for only five minutes: I obtain a buff yellow coloured precipitate which, on standing, gives me the same metallic precipitate I observed earlier.

I expected that Fehling's solution will give a red precipitate of $\ce{Cu2O}$ but it looks like the formaldehyde seems to be oxidizing it all the way down to metallic copper. Also, what is the reason for a yellow precipitate to form rather than a red one?

I could not take any pictures: We're not allowed to carry phones/cameras into the lab.

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  • $\begingroup$ Yeah sometimes you even end up in a blue-green solution, with no precipitate, something you'd confidently conclude to be a negative result. But later I was told by my teacher that it actually is an aldehyde and that turning green is a sign of a reaction undergoing. Net result: I scored poorly in that test -_- $\endgroup$ Feb 5, 2020 at 14:02

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In the reaction with aldehydes, Fehling's solution produces $\ce{Cu2O}$ as an orange or yellow precipitate. I don't know why the color is not always the same. The oxidation of an aldehyde produces an acid. And an acid usually cannot be further oxidized.

There is one exception: methanal $\ce{HCHO}$ that can be oxidized to formic acid $\ce{HCOOH}$, and then into $\ce{CO2}$. That is why Fehling's reagent produces metallic copper $\ce{Cu}$, $\ce{CO2}$ and no acid when reacting with $\ce{HCHO}$.

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  • $\begingroup$ Alright that does make sense. I did not consider the further oxidation. However, I don’t think I saw any bubbles of CO2 emerging. Could you please also provide a reference/reaction mechanism? $\endgroup$ Feb 5, 2020 at 11:15
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    $\begingroup$ By all means, precipitated copper will not appear black. Sorry, the explanation is not correct. Please Google Oxidation of formaldehyde by copper(II) in aqueous solution byJ. J. BYERLEAYN D W. K. TEO. It specifically says that no gas evolution seen and they rule out the oxidation of formaldehyde to CO2. $\endgroup$
    – AChem
    Feb 5, 2020 at 12:32
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    $\begingroup$ Who said there was a black precipitate ? Nobody ! $\endgroup$
    – Maurice
    Jun 6, 2021 at 10:24

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