Test for Reducing Sugars

Aldoses and ketoses (i.e., most monosaccharides) are classed as reducing sugars because their free aldehyde or ketone group can chemically reduce other compounds. In Benedict's test, the sugar sample is heated with an alkaline solution of copper(II) sulphate (Benedict's solution, which starts out blue). If a reducing sugar is present, its aldehyde or ketone group reduces the blue Cu2+ ions to Cu+ ions, which precipitate out as brick-red copper(I) oxide; in the same reaction, the sugar's aldehyde/ketone group is itself oxidised to a carboxyl group (-COOH). The colour of the final precipitate depends on how much reducing sugar was present: the solution stays blue if there is no reducing sugar, turns green with a trace amount, yellow/orange with a moderate amount, and a full brick-red precipitate forms when a large amount of reducing sugar is present - so Benedict's test can give a rough, semi-quantitative estimate of sugar concentration, not just a yes/no result. Notably, **sucrose is NOT a reducing s …