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.