Question Video: Determining the Relative Amounts of Reducing Sugars in Different Samples by the Color of Benedict’s Solution Biology • First Year of Secondary School

Video Transcript

Benedict’s solution is used to test samples for the presence of sugars. The results from multiple tests are shown in the diagram. Which of the following options correctly describes the proportions of reducing sugars in the samples shown in the diagram? (A) X has moderate amounts of reducing sugars, Y has a high proportion of reducing sugars, and Z has small traces of reducing sugars. (B) X has a high proportion of reducing sugars, Y has moderate amounts of reducing sugars, and Z has small traces of reducing sugars. (C) X has small traces of reducing sugars, Y has a high proportion of reducing sugars, and Z has moderate amounts of reducing sugars. (D) X has small traces of reducing sugars, Y has moderate amounts of reducing sugars, and Z has a high proportion of reducing sugars.

Sugars, or carbohydrates, are a large part of our diets and are essential reactants in cellular respiration. An example of a sugar you will commonly consume is glucose, which is a simple sugar also known as a monosaccharide. Monosaccharides are reducing sugars and can be tested for using Benedict’s reagent. Let’s take a look at how Benedict’s reagent is a blue solution that contains a mixture of chemicals, including copper oxide. When Cu2+ ions in this copper oxide are exposed to reducing sugars, they gain an electron to become Cu+ ions. This reaction forms a red precipitate.

The more reducing sugars present, the more red precipitate is formed. So, sample Z must have a lot of reducing sugars present, because the solution has turned a brick-red color. Moderate amounts of sugar will produce a yellow colored solution. And small amounts of reducing sugars will only slightly change the color of the Benedict’s reagent from blue to green. So, from sample X to sample Z, we can see the amount of reducing sugars present is increasing.

Using this knowledge, let’s look back at our answer choices. The option that correctly describes the proportions of reducing sugars in the samples is (D). X has small traces of reducing sugars, Y has moderate amounts of reducing sugars, and Z has a high proportion of reducing sugars.