In Fehling's Test

The free sugar group (aldehyde or ketone) reduces the Cu  (cupric) ions in  an alkaline environment  to form Cu₁₀ (cuprous oxide) and the sugar is itself oxidized to a carboxylic acid (mixture  of sugar  acids). The  Cu₂0 is  red  and  precipitates. Carbohydrates that react  in  basic solution with  oxidizing agents are classified as reducing sugars. In basic solution, all monosaccharides whether aldoses or ketoses, are reducing sugars. Simple sugars can act as reducing agents because the aldehyde or ketone group is readily oxidized  to carboxylic  acid.