# Fehling's solution turning yellow and then black?

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.

• 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 -_- – Sir Arthur7 Feb 5 '20 at 14:02

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 usually an acid cannot be more oxidized. There is one exception : methanal $$\ce{HCHO}$$. Because it can be oxidized in formic acid $$\ce{HCOOH}$$, and then into $$\ce{CO2}$$ . That is why Fehling's reagent produce metallic copper $$\ce{Cu}$$, $$\ce{CO2}$$ and no acid when reacting with $$\ce{HCHO}$$.