Victory for the ACT Student Text 15e

308 • S CIENCE

PASSAGE II

Simple carbohydrates ring structure called a di saccharide. Familiar sugars include the monosaccharides glucose (the basic fuel of cells) and fructose (found in fruits) and the disaccharides sucrose (table sugar) and lactose (the sugar in milk). Benedict’s solution is a chemical reagent commonly used to detect the presence of reducing sugars. The reducing sugars include all monosaccharides such as glucose and some disaccharides such as lactose. Sucrose is not a reducing sugar. The principle of Benedict’s test is that when reducing sugars are heated in the presence of an alkali, they reduce the cupric compounds (Cu ++ ) present in the Benedict’s reagent to cuprous compounds (Cu + ) which are precipitated as insoluble red copper oxide (Cu 2 O). The color of the precipitate gives an approximation of the quantity of sugar present in the solution. Table 1 Result of Benedict’s Test Concentration of Reducing Sugar (by weight) Blue (unchanged) 0.0% Green 0.1%–3.5% Yellow 3.6%–5.0% Red-Brown >5.0% Experiment 1 A group of students prepared test samples by measuring 40 drops of seven different liquids—distilled water, 10% sucrose solution, 6%-glucose solution, heavy cream, whole milk, no-fat milk, and unsweetened soy milk—into pre-labeled test tubes. To each test tube, the students added 10 drops of Benedict’s solution. The test tubes were agitated and then placed in racks and immersed in a water bath at 95°C for 10 minutes. The students observed any color changes in the solutions and interpreted the changes using Table 1. The students recorded the results in Table 2. Table 2 Substance Initial Color Final Color Distilled Water Blue Blue Sucrose Solution Blue Blue Glucose Solution Blue Red-Brown Heavy Cream Blue Yellow Whole Milk Blue Yellow No-Fat Milk Blue Yellow Unsweetened Soy Milk Blue Blue Experiment 2 Lipids are a class of organic compounds that are fatty acids or their derivatives. They are soluble in organic solvents but insoluble in water. Sudan red is a dye that attaches to lipids. The students performed a second experiment in which 1 milliliter of each of the liquids used in Experiment 1 was pipetted into pre-labeled test tubes. Two drops of Sudan red were added to each test tube, and the students recorded the results in Table 3. can form either a single ring structure called a mono saccharide or a double