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What causes the result of a Benedict's test to be positive?

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A positive Benedict's test is the result of the presence of reducing sugars. A positive Benedict's test will cause the solution used in the test to form reddish precipitate, according to Harper College.

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Full Answer

To perform the test, a sample of the solution to be tested is combined to make Benedict's reagent. This solution is then heated in a boiling water bath for three minutes. The formation of a reddish precipitate indicates a positive test for reducing sugars. In addition to this test, there is also a solution called Benedict's quantitative reagent, which determines how much reducing sugar is present. This test will form a white precipitate on a positive test.

Reducing sugars that can be detected by Benedict's test include all monosaccharides and many disaccharides. This includes lactose and maltose, according to Wikipedia. The test will also detect aldehydes and alpha-hydroxy-ketones. Although ketose fructose is not strictly a reducing sugar, it can give a positive test because the base of Benedict's reagent will convert it into the aldoses glucose and mannose.

Benedict's test is named after the American chemist, Stanley Rossiter Benedict. Benedict's reagent is made from anhydrous sodium carbonate, sodium citrate, and copper (II) sulfate pentahydrate. This reagent can also be used in place of a Fehling's solution, a chemical test used to differentiate between a water-soluble carbohydrate and ketone functional groups.

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