Chemistry Aldehydes & Ketones UHS 2024
PMDC Verified Question 2 of 94
The blue colour of Fehling solution is changed to red when warmed with an aldehyde due to formation of which of the following?
A
\( NO_2 \)
B
\( AgO \)
C
\( Cu_2O \)
D
\( SO_2 \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \( Cu_2O \)
Concept:

Fehling's test identifies aldehydes via a specific reduction of metal ions.

Solution:

  • Fehling's solution contains Copper(II) ions (\( Cu^{2+} \)), which give the solution a distinct deep blue color.


  • When warmed with an easily oxidizable aldehyde, the aldehyde forces electrons onto the copper.


  • The Copper(II) is reduced to Copper(I).


  • This forms Copper(I) oxide (\( Cu_2O \)), which is highly insoluble and precipitates out of the solution as a characteristic brick-red solid.


Why other options are incorrect:

\( AgO \) (Silver oxide) is related to Tollens' test, which produces elemental silver, not red oxide. \( NO_2 \) and \( SO_2 \) are gases unrelated to this specific test.

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