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I think you are referring to the test using Fehlings solution. Fehlings solution oxidises aldehydes and ketones and formic acid and is in turn reduced. The red precipitate is the copper(I) oxide formed by reduction of the copper(II) complex found in Fehlings solution. Acetic acid is not readily oxidised and so there is no precipitate. See link for more information on what the complex is in Fehlings solution and how it is prepared.
When Fehling's solution is added to an aldehyde it turns brick red
Fehling's solution must be freshly prepared because there is a tendency for it to oxidize if not used right away
When the two Fehling Solutions A and B are mixed, a deep blue solution containing a complex cupric ion is formed. On interaction with reducing compounds such as aldehydes or sugars, the copper is reduced to the univalent stage, and a red, yellow, or yellowish green precipitate is formed.
No colour
I think you are referring to the test using Fehlings solution. Fehlings solution oxidises aldehydes and ketones and formic acid and is in turn reduced. The red precipitate is the copper(I) oxide formed by reduction of the copper(II) complex found in Fehlings solution. Acetic acid is not readily oxidised and so there is no precipitate. See link for more information on what the complex is in Fehlings solution and how it is prepared.
Fehling solution "A" is copper sulphate solution and Fehling solution "B" is a solution of sodium potassium tartrate and NaOH.
When Fehling's solution is added to an aldehyde it turns brick red
Fehling's solution must be freshly prepared because there is a tendency for it to oxidize if not used right away
The Fehling A solution contain copper sulfate.The Fehling B solution contain sodium potassium tartrate and sodium hydroxide.
When the two Fehling Solutions A and B are mixed, a deep blue solution containing a complex cupric ion is formed. On interaction with reducing compounds such as aldehydes or sugars, the copper is reduced to the univalent stage, and a red, yellow, or yellowish green precipitate is formed.
Aldehyde are oxidised to the corresponding carbonic acid by F.'s or T.'s agents. Ketone can not be oxidised.
No colour
When Fehling A & B are mixed in equal quantities... Fehlings Reagent is formed which is DEEP BLUE in colour Hope that answers your question! :)
Fehlins test is done to identify aldehides. Fehlins solution is used for this test. If aldehides are added to this soluton, a red precipitate is formed.
It is a solution which is transparent in colour.
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