Starch is a polymer of the monosaccharide glucose, why does it test negative with the Benedict’s Reagent when glucose tested positive?
Benedict's reagent is a solution that has sodium citrate, sodium carbonate and cupric sulphate pentahydrate. It is used to test for the presence of reducing sugars. These include monosaccharides like glucose and disaccharides like sucrose.
The aldehyde group of the sugar is oxidized and the cuprous ion in the reagent is reduced to form cuprous oxide. When a reducing sugar is added to Benedict's reagent, which is a blue colored solution, and the solution boiled for a few minutes and allowed to cool, a precipitate is formed that could be red, green or yellow in color.
Starches are polymers of monosaccharides, but they cannot be detected using Benedict's reagent as there are very few reducing sugar moieties that exist only at the ends of the carbohydrate chains.