Dear student,

Fehling's test takes advantage of the ready reactivity of aldehydes by using the weak oxidizing agent cupric ion (Cu²⁺) in alkaline solution. In addition to the copper ion, Fehling's solution contains tartrate ion as a complexing agent to keep the copper ion in solution. Without the tartrate ions, cupric hydroxide would precipitate from the basic solution. The tartrate ion is unable to complex cuprous ion Cu⁺, so the reduction of Cu²⁺ to Cu⁺ by reducing sugars results in the formation of an orange to red precipitate of Cu₂O.

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