Colors range from green yellow orange to red. How does the sugar concentration and Cu2O concentration cause this? I know Cu2O is red. Why is there a green or yellow color instead of just red or some kind of red?


Benedict's Reagent is just simply $\ce{Cu^2+}$ ions in the form of copper citrate. When Benedict's Reagent finds an aldose (a sugar with an aldehyde group), it can oxidize the aldose to a carboxylic acid. For example:

D-Glucose being oxidized

Since D-Glucose is oxidized, Cu is reduced to a red precipitate ($\ce{Cu2O}$). The colors range form green to red because the original Copper Citrate ($\ce{C6H8Cu2O7^4+}$) is blue in color.

Source: Quora

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  • $\begingroup$ how does it become green if cu2o is red? $\endgroup$ – Soapkz Sep 15 '17 at 13:58
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    $\begingroup$ "The compound can appear either yellow or red, depending on the size of the particles." en.wikipedia.org/wiki/Copper(I)_oxide yellow + blue =... $\endgroup$ – Mithoron Sep 15 '17 at 14:21
  • $\begingroup$ How does that relate to concentration? And what about green? $\endgroup$ – Soapkz Sep 16 '17 at 11:37
  • $\begingroup$ Color theory - Yellow Copper(I) Oxide + Blue Copper Citrate = Green solution $\endgroup$ – JSCoder says Reinstate Monica Sep 16 '17 at 15:58
  • $\begingroup$ So at higher concentrations copper oxide particles clump together? $\endgroup$ – Soapkz Sep 17 '17 at 12:49

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