Exercise 5

C HAPTER 5 | The Composition of Living Things Understanding Organic Molecules 5 Procedure 1 Benedict’s Test Benedict’s reagent , containing sodium citrate, sodium bicarbonate, and copper sulfate, is used to identify reducing sugars. These sugars can be monosaccharides or sometimes disaccharides that possess a free aldehyde or ketone functional group. Reducing sugars have the ability to reduce (remove oxygen or add hydrogen to) oxidizing agents, such as the copper compound in Benedict’s solution. Polysaccharides and some disaccharides, such as sucrose, are not reducing sugars because they do not possess a free aldehyde or ketone functional group and there- fore do not donate protons to oxidizing agents. Benedict’s reagent is blue because of the cupric, or copper ions. When the reagent is mixed with a solution containing reducing sugars and then heated, a colorful precipitate may form in the bottom of the test tube. If no reducing sugar is present, the solution will remain blue and not form a pre- cipitate. In the presence of a very small amount of reducing sugar it will produce a green precipitate, in a low amount it will produce a yellow precipitate, in a moderate amount it will produce a yellowish-orange precipitate, in a high amount it will produce an orange precipitate, and in an extremely high amount it will produce a brick-red precipitate. 1 Place approximately 250 mL of tap water into a 500 mL beaker. Place the beaker on a hot plate, and begin to heat the water. When the water begins to boil, set the tempera- ture to medium. 2 Procure 10 clean test tubes and wax pencils or Sharpies. Label the test tubes 1 through 10. Materials ❏ Hot plate ❏ 500 mL beaker ❏ Test tube rack ❏ Test tube holder ❏ Test tubes (10) ❏ Wax pencils or Sharpies ❏ Metric ruler ❏ Dropper or transfer pipette ❏ Benedict’s reagent ❏ Tap water ❏ Test solutions: b distilled water b clear diet soda b clear nondiet soda b pineapple juice b onion juice b potato juice b milk b glucose solution b sucrose solution b corn syrup Benedict’s reagent is corrosive. Wear gloves. If it splashes onto your skin, wash the area immediately with soap and water. WARNING !