Benedict's test is a simple chemical test to detect reducing sugars. Reducing sugars are carbohydrates that have an available aldehyde or ketone functional group in their molecular structure. These monosaccharides include glucose and fructose and disaccharides such as lactose and maltose. Glucose is called a reducing sugar because it has a free aldehyde group and can donate electrons to other compounds, this process is reduction. Fructose is also a reducing sugar because it contains a free ketone group.
Reducing sugar is sugar that can act as a reducing agent. A reducing agent reduces the oxidation number of an element or compound, for example, copper can be reduced from Cu2+ to Cu+. All reducing sugars can be detected by Benedict's test. All monosaccharides are reducing sugars. Examples are lactose, maltose, glucose, fructose, galactose, ribose, and xylose.
Figure 1: Glucose in its linear form. The aldehyde group is highlighted from Labster’s theory page.
Note: Regular table sugar is not a reducing sugar. It has no free aldehyde or ketone groups that can be rearranged to form aldehyde groups in solution.
Read on to find out why this can be a boring topic for teachers and students, five suggestions for changing the situation, and ideas for why virtual labs can make things easier.
Figure 2: A snippet from Benedict's test simulation by Labster showing the necessary setup.
There are three main reasons why Benedict's test can be difficult for even the most diligent of students.
Figure 3: An extract from Benedict's test simulation by Labster showing different monosaccharides that have been examined.
We teach students about Benedict's test and its use in detecting reducing sugars, but ignore the challenges associated with this test, such as the use of visual aids in diagnosis are not very accurate but can be used presumably, false positive tests can also be given because the presence of ascorbic acid, homogentisic acid or other reducing agents, therefore does not completely confirm diabetes, and complex carbohydrates such as starch do not respond positively until they are broken down by heating or digestion before a positive reaction will occur.
Benedict's test is used to determine if reducing sugars are present in a sample using a reagent called Benedict's reagent. The reagent is a basic solution containing a copper citrate complex with Cu2+ ions. When the reagent reacts with the aldehyde group of reducing sugar, Cu2+ ions are reduced to Cu+ ions, forming a red precipitate of copper oxide. At reduced quantities, these deposits appear orange or green. The chemical reactions of the main functional groups are shown below.
Figure 4: Simple reaction of reducing sugar with Benedict's reagent from Labster’s theory page.
Benedict's solution is blue. Because the test is based on color change, the color of Benedict's solution is very important for the test. The CuSO4.5H2O salt (copper sulfate pentahydrate) is the source of the blue color due to the presence of copper(II) ions. Copper(II) can sometimes be changed to copper(I), and sodium citrate is used to stabilize the copper ion by complexing it. Sodium carbonate is used to maintain the alkaline environment required for the redox reaction during the test. Water is only added as a solvent for the reaction.
In chemistry, control is a way to validate the results of your experiments. The negative control contains all the reagents used in the experiment except the material to be detected. Therefore, the negative control should give a negative result while the positive control contains the material to be detected, so the positive control should return a positive result.
The negative control validates the positive result and the positive control validates the negative result. Without comparing the unknown sample with the control, we cannot ascertain whether our results are due to the presence or absence of the compound we tested or an error in the procedure. Example: When testing a reducing sugar, the negative control is water and the positive control is glucose. Water does not contain reducing sugars, so it gives a negative result. Glucose is a reducing sugar, so there should be a positive result. If the control does not produce the expected results, we can conclude that there is an error in the process.
Figure 5: Negative and positive control results for Fehling's reducing sugar test from Labster’s theory page.
Now that you have the basics right, let's review how you can make Benedict's exam classes engaging, easy, and fun for you and your students.
In 1674, Thomas Willis tested the sweetness of the urine of diabetics. Dobson demonstrated in 1715, through a technique of fermentation, that this sweetness was due to the presence of sugar. In 1815 Vogel discovered the reducing property of glucose solutions in the presence of alkali compounds of metal salts. About 100 years later, Stanley R. Benedict developed a chemical test for urine sugar that was almost universally used throughout North America. It took nearly 250 years from urine tasting to Benedict's test.
These are the equipment and materials for Benedict's test.
test tube and test tube rack
tripod and Bunsen burner
500ml glass glass
sample for testing
Figure 6: Device setup with hot water bath.
The procedure for Benedict's test is:
Pour approximately 1mL of Benedict's Reagent into a clean, dry test tube.
Add 2- 3 drops of the liquid sample to the test tube.
Heat the contents of the jar in a hot water bath for about 3 minutes.
The formation of a red precipitate of copper oxide indicates the presence of reducing sugars. Small amounts of reducing sugar appear as a green or orange precipitate.
To determine the presence of glucose in the blood and urine, the detection of excess glucose means diabetes.
A good mnemonic for examples of monosaccharides is MaL X-ray GRaFt where lowercase letters are excluded and 'G' stands for two monosaccharides.
M - Maltose
L - Lactose
X - Xylose
G - Galactose
G - Glucose
R - Ribose
F - Fructose
A unique way to teach Benedict's test is through a virtual lab simulation. At Labster, we are dedicated to providing fully interactive state-of-the-art lab simulations that use gamified elements such as storytelling and scoring systems in an immersive, 3D world.
Check out Labster's Benedict's Test Simulation, which helps students learn about Benedict's test in sugar detection through active inquiry-based learning. In the simulation, students embark on a mission to help some street basketball players understand how the carbohydrate foods they eat are tested using Benedict's test.
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