The term 'reducing sugars' denotes carbohydrates that give positive tests with Benedict's reagent, a solution of the citrate complex of copper (II) sulfate which oxidizes aliphatic aldehydes. Of monosaccharides, aldoses and ketoses both test positively (ketoses because of keto-enol tautomerism). Carbohydrates containing a free hemiacetal group are reducing sugars because the hemiacetal is in equilibrium with the open chain form. Fehling's solution and Tollen's reagent are also used as tests for reducing sugars.