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Whilst doing tests on two unknown solutions, I used Fehling's test. The solutions could be a mixture of compounds or just one compound dissolved in water/ another solvent - I don't know. Upon adding Fehling's solution both solutions (which were originally colourless) turned a blue colour. I then heated the solutions very gently for about 5 minutes and after this time one had completely formed a red precipitate, whereas the other still had the blue solution left in it (while forming some red precipitate). After waiting for about 20 minutes both solutions had completely formed a brick red precipitate and were the same. I assume this is to do with rate of the oxidation reaction occurring. Is there any explanation as to the rate of reaction in each solution or an explanation which is not to do with rate at all perhaps?

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  • $\begingroup$ Is that 20 minutes of heating? or just sitting on the bench? $\endgroup$ – LDC3 May 10 '15 at 15:33
  • $\begingroup$ It's 20 minutes of warming at around 80 degrees Celsius. $\endgroup$ – Resquiens May 17 '15 at 14:25
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The reason is thus:

For aldoses (like glucose), the copper sulfate in the reagent easily oxidizes it into the carboxylic acid forming copper oxide (a precipitate).

For ketoses (like fructose), the ketone cannot be oxidized, so there is no color change. Over time at elevated temperature, the α-hydroxy ketone isomerizes to a α-hydroxy aldehyde. This aldehyde is then oxidized to the carboxylic acid, producing a red precipitate.

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