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5 Ways to Make Fehling’s Test A More Approachable Topic to Students

Akanksha Saxena
High School
Teaching with Labster

Introduction: Fehling’s Test

The food we consume is a complex mixture of several macromolecules like sugars (reducing and non-reducing), carbohydrates, lipids, proteins, etc. All these together provide a surplus source of nutrition and energy for running the body’s metabolism. The human body is designed in such a way that it requires a proportionate diet where each of these molecules is assigned a specific role. 

These different types of macromolecules are chemically diverse and have unique compositions. One of these molecules is “sugar”. Sugar is a type of carbohydrate. Simple sugars belong to different classes like monosaccharides (glucose, fructose, galactose) and disaccharides (sucrose, lactose, maltose). Sugars are also classified based on their reducing nature; “reducing sugars” and “non-reducing sugars”.

When teaching this aspect of Biology to young learners, the topics aren’t usually restricted to their knowledge of one subject. Somehow the chemistry also creeps in… And this is where biologists sometimes get petrified! The fate of Fehling’s test is the same!

Though it’s a very simple type of test to differentiate between reducing and non-reducing sugars, many students fail to comprehend the science behind it. Students introduced to this test often find it difficult to learn its details, underlying principles, and real-world applications. Even educators dealing with the topic sometimes fail to deliver the essence of this topic and its crux. 

We, at Labster, understand the complexities of this biochemical test. This article can provide some help as it attempts to identify the major issues encountered by students while studying this topic. It also lists practical solutions teachers and educators can incorporate in their next class. By the end, we’ll convince you why a virtual lab simulation will prove useful for your students and for you as an educator to deliver concepts more efficiently.

Why can Fehling’s Test be tricky to teach or learn?

There are 3 reasons why students dread and get confused about the topic of Fehling’s Test. Acknowledging these blocks is the first step toward making the topic more approachable. 

1. Failing to comprehend the test's underlying premise

This is one of the main reasons why most students don’t understand the working of Fehling’s test and other biochemical tests performed for food macromolecules. When the underlying principle is unclear, the test becomes less logical and more theoretical to students. The ‘why’ (reason for performing the test) and ‘how’ (the underlying principle of the test) of each test performed for different macromolecules must be thorough to young learners.

2. The core concepts about macromolecules structures are weak

Directly teaching about the Fehling test in your next class might not work Many students don’t know the basic difference between “reducing and non-reducing sugars”, “aldehydes and ketones”, “aliphatic and aromatic aldehydes”, etc. Basic knowledge about sugars, their structure, bonds involved, etc is a prerequisite before one moves on to complex topics like biochemical testing, reagents used for tests, etc. 

3. Unaware of its practical uses

Learning a biochemical test seems worthless to students when they don’t know how to exploit it in their future research or industry job or businesses. Fehling’s test is often taught to students without educating them about its practical utility. This makes the learning journey dull and non-conducive.

5 ways to make Fehling’s Test a more approachable topic to understand

To address the issues encountered while teaching this topic, educators can engage the under-listed solutions in their classes. These can decode many different aspects of Fehling’s Test. Not only can they make teaching easier for educators like you, but they will also make lessons clearer and easier to assimilate for your students.

  1. Strengthen the core concept of sugars and their structures

This is our foremost advice to all the educators dealing with Fehling’s test. Designing biochemical tests that exploit the basic biochemistry of sugars at the molecular level necessitates the need for strong core concepts. You can begin with the underlisted examples in your next class.

  • Different types of food macromolecules (You can use the Introduction to Food Macromolecules simulation from Labster)

  • Basis of differentiating different food macromolecules

  • Introduction of carbohydrates

  • Difference between carbohydrates and sugars

  • Different types of simple sugars (Monosaccharides and Disaccharides)

  • Different types of functional groups (Aldehydes and Ketone groups)

  • Different types of aldehydes (Aliphatic and Aromatic)

  • Different types of sugars (Reducing and Non-reducing sugars)

Catering to these topics before moving to the explanation of Fehling’s test can ease the work for you. Additionally, it will encourage students to raise questions in a more open-ended manner during classroom teaching sessions related to “basic science.”  

fehling's test virtual lab

Figure: Snippets from Fehling’s Test simulation by Labster showing the virtual lab handling for Fehling’s test. It is available for High School classes.

2. Teach them how to approach a problem scientifically

Fehling’s test is one of the many biochemical tests used in both academic research and the R&Ds of industries. Teaching your students how to scientifically approach a problem can help them reason out where “Fehling’s test can be applied” and “where it shouldn't be”. You can quote the under-listed example to showcase why scientifically approaching a problem can save both time and effort.

Example: Fehling’s reagent is a weak oxidizing solution and thus it works only for aliphatic aldehydes but not for aromatic aldehydes. Compared to Fehling’s reagent, Tollen’s reagent is a stronger oxidizing solution capable of oxidizing aliphatic and aromatic aldehydes. So, if you mistakenly use Fehling’s test for benzaldehyde, it will never work, no matter how many times you try it!

This will help them recognize the importance of scientific methods. If one does their background research work well and meticulously plans their experiments, they can avoid unnecessary hurdles that usually stagger lab work. 

Further, teaching them how to set the experiment, use controls (both positive and negative controls), the meaning and importance of these controls, and drawing out conclusions by comparing the sample with controls can advance their scientific aptitude. You can also use The Scientific Method simulation and the Experimental Design simulation from Labster where similar concepts are explained in detail.

Positive and Negative Controls benedict's theory

Figure: Positive and Negative controls in Fehling’s test. Image Source 

3. Demonstrate the technique

Students are more likely to remember tests and their specific details when they get hands-on experience with lab procedures. We understand that it’s not always possible for educators to conduct individual practice sessions for all the experiments. In such cases, we recommend at least demonstrating the experiment. Since Fehling’s test is based on a “chromogenic change (color change; blue to brick red)”, students might find it quite interesting when they observe the color change in front of their eyes.

You can use different food items like table sugar, potato chips, oats, popcorn, avocado, apple juice, milk, candies, etc which are rich in carbohydrates and sugars to conduct a practical class testing reducing sugar’s presence in them. Watch out for your students as the color changes of Fehling’s test enthuse them in the class! 

demonstration of fehling's test

Figure: Demonstration of Fehling’s test. Image Source

4. Simplify, don’t complicate!

Educators must simplify the science behind technical experiments. It's crucial to explain why the color in this experiment changes. If you simplify that for your students, half of the job is done! Take leads from the underlisted points and how to simplify them for your students.

  • Explain why Fehling’s solution is blue. The initial color of the solution is blue because it contains copper ions. Fehling's solution is composed of 2 components; Fehling’s A and Fehling’s B solutions. Fehling’s A is the copper sulfate (CuSO4) solution that gives the blue color.  Fehling’s B is an uncolored liquid consisting of potassium sodium tartrate (also called ‘Rochelle salt’) and a strong alkali (mostly sodium hydroxide, NaOH). 

  • Explain why Fehling’s solution is always prepared fresh.  Fehling’s reagent tends to deteriorate over time. Since copper tartrate complexes are unstable, Fehling’s A and B solutions (quite stable) are prepared and stored separately. And they are mixed in equal proportions before use. 

copper tartrate complex

Figure: Structure of Copper tartrate complex (unstable). That’s why Fehling’s reagent is always freshly prepared. Image Source

  • Explain how Fehling’s test works. Fehling's reagent contains Cu2+ ions and tartrate ions. When this reagent encounters an aliphatic aldehyde, the Cu2+ ions are reduced to Cu+ ions. Simultaneously, the aldehyde functional group is oxidized to COO-. The change is visualized as the blue solution changes to a brick-red precipitate (cuprous oxide). Since all reducing sugars have a free aldehyde group, they show a positive Fehling test.

reaction of fehlings test

Figure: Reaction of Fehling’s Test. Image Source

5. Use virtual lab simulations

Fehling’s test is a practical technique used for the detection of reducing sugars in food-based research labs and industries. When teachers and educators aren’t able to demonstrate the experiment in their classes, theory lessons can be quite taxing for students.

We at Labster understand the issues faced by both students and teachers. Therefore, we encourage modern-day educators to make the most of Fehling’s Test simulation from Labster. It takes your students into a virtual world where they can understand how reducing sugars are different from non-reducing ones, how Fehling’s reagent is prepared, how positive and negative controls are assigned, etc. It also helps your students to actively engage in the experiment using different food items like milk, groundnut oil, grape juice, egg whites, wheat solution, sucrose, etc.

Your students don’t have to struggle anymore as our interactive Fehling’s Test simulation and gamification elements will save the day for you. Using this way of active and immersive teaching, our virtual learning platform takes an advent in the field of Science to make the upcoming scientists thorough with the “basics of their respective subjects”. 

Fehling’s Test GIF

Figure: A snippet from Fehling’s Test simulation by Labster where your students can test the presence of reducing sugars in different food samples. It is available for High School classes.


You can learn more about Fehling’s Test simulation here or get in touch to find out how you can start using virtual labs with your students.

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