5 Ways to Make Tollen’s Test A More Approachable Topic

Introduction: Tollen’s Test

Organic compounds enjoy a lot of attention in industry R&D as well as academic research. Out of all the chemical compounds produced at the industrial scale, almost 35% are organic compounds. Organic molecules with carbonyl groups are of different types depending on where the trigonal planar carbon of the carbonyl group attaches to two other substituents. The commonly known organic molecules with carbonyl groups are aldehydes, esters, ketones and carboxylic acids. 

Each of these molecules with a carbonyl group presents different properties. Several organic carbonyl compounds find widespread industrial applications across different domains. From “use of formaldehyde (an aldehyde) in the plastic manufacturing industry” to the “use of carbonyl compounds like diacetyl, acetoin,acetone, methyl ketones and 2-butanone (ketones) and acetaldehyde (an aldehyde) in the dairy industry especially in the flavoring of cheeses, yogurt and fermented dairy products”, these organic carbonyl compounds find distinguished and widespread usage. (Source)

Since the nature of these carbonyl compounds varies, biochemical testing for identifying one from another gains particular interest in both academic research and industrial scale. One such test for distinguishing aldehydes from ketones is Tollen’s test. Tollen’s reagent is a stronger oxidizing solution than Fehling’s reagent. It is capable of oxidizing both aliphatic and aromatic aldehydes while Fehling’s test works only for the aliphatic aldehydes. Both tests yield negative results for ketones. 

Both tests are widely used to check for the presence of reducing sugars. Reducing sugars are a type of simple carbohydrate. They form one of the most important food macromolecules. Food macromolecules are chemically diverse and have unique compositions. 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”.

Though Tollen’s test is a very simple type of test to differentiate between reducing and non-reducing sugars, many students fail to comprehend the science behind it. Most often students are introduced to the idea of organic chemistry in their high school Chemistry classes. The topic seems interesting and practically very useful. Even after that, students are often seen hassling with the underlying basic concepts like the principle of Tollen’s test.

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 whilst studying this topic. It also lists practical solutions that teachers and educators can incorporate into their next class. By the end, we’ll convince you why a virtual lab simulation will prove useful not only for your students but also for you as an educator to deliver concepts more efficiently.

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

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

1. Core concepts of organic chemistry are weak

This is one of the main reasons why most students don’t understand the working of Tollen’s test. Basic organic chemistry topics are sometimes very confusing for students. The different chemical structures, like Fischer and Haworth projections and Lewis structures of aldehyde and ketone families confuse the students. This often leads to a lack of understanding of the different roles and characteristics of aldehydes and ketones. When the underlying concepts are not clear, the test becomes less logical and more theoretical to students. Not knowing the ‘why’ (reason for performing the test) and ‘how’ (the underlying principle of the test) of Tollen’s test can lead to a situation where students try to either rote learn the topic or ignore it. 

2. Difficulty in following the ideas and symbolic representations

Many students find it hard to follow the ideas of valency, electron transfer, oxidation-reduction reactions, acidic versus basic nature of substance or solution, ions and atoms, coordinate complexes, weak versus strong oxidizing/reducing agents, etc. Additionally, they often struggle to comprehend the differences between chemical reactivity and the nature of aliphatic aldehydes and aromatic aldehydes. This creates confusion in the proper selection of the biochemical tests.

3. Unawareness of its practical uses

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

5 ways to make Tollen’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 Tollen’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, their types, and 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 more open-ended manner during classroom teaching sessions related to “basic science”.  

Figure: A snippet from the Tollen’s Test simulation by Labster can learn about the requirements for conducting the test. It is available for School and University/College classes.

2. Teach them how to select a test appropriately

Tollen’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 approach a problem scientifically can help them reason out “where Tollen’s test can be applied” and “where it shouldn't be”. This will help them in recognizing the practical applicability of techniques. It will also boost their confidence to translate theory lessons into practical experiments. 

Example: Tollen’s reagent is a stronger oxidizing solution than Fehling’s reagent. This is why it can work for both aliphatic and aromatic aldehydes. On the other hand, Fehling’s reagent is a weaker oxidizing solution that can only oxidize aliphatic aldehydes. And both the reagents don’t work for ketones. 

• Use of Tollen’s or Fehling’s test: If you use Tollen’s or Fehling’s test for ketones, it’s never going to work, no matter how many times you try the test! (Rationale: Both Tollen’s and Fehling’s tests detect the presence of an “aldose group”. And this is why they confirm the presence of reducing sugars which possess a characteristic aldose group. On the other hand, non-reducing sugars have a “ketone group” and thus their presence can’t be confirmed by these tests.) Both the tests exploit the common knowledge that “aldehydes are readily oxidized while ketones aren’t”.
• Use of only Tollen’s test: So, if you use Tollen’s test for benzaldehyde (an aromatic aldehyde), it’s going to work in contrast to Fehling’s test that fails for aromatic aldehydes. (Rationale: Tollen’s reagent is a stronger oxidizing agent than Fehling’s reagent and thus successfully oxidizes aromatic aldehydes in addition to aliphatic ones.)

Explaining such examples can help your students recognize the importance of scientific methods in planning experiments. If one does their background research work well and meticulously plans their experiments, they can avoid unnecessary hurdles that usually stagger lab work. You can also use The Scientific Method simulation and the Experimental Design simulationfrom Labster where similar concepts highlighting the importance of controls (positive and negative), replicates (biological and technical), etc are explained in detail.

Figure: A snippet from Tollen’s Test simulation by Labster showing the lab setup for the test. It is available for School and University/College classes.

Discover Labster's Tollen’s Test virtual lab today!

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 Tollen’s test is based on a “chromogenic change (color change; colorless to silver)”, students might find it quite interesting when they observe the color change in front of their eyes.

Figure: A schematic representation of Tollen’s reagent preparation, test, and results (both positive and negative). A negative test is confirmed by a colorless solution while a positive test is confirmed by a silver mirror/gray precipitate formation. Image Source

You can use different food items like milk, groundnut oil, grape juice, egg white, wheat solution, sucrose, potato chips, oats, 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 Tollen’s test enthuse them in the class! 

Additionally, you can increase participation by asking your students to bring a variety of foods from home that they believe to be high in sugars and carbohydrates so that you can include them in the experiment.

Figure: A snippet from Tollen’s Test simulation by Labster where your students can engage in a virtual experiment and perform the test for different food items. It is available for School and University/College classes.

4. Simplify the steps and underlying principle of the test

Educators must simplify the science behind technical experiments. It's crucial to explain why the color in this experiment changes. Take leads from the underlisted points about how to simplify the science behind this test to your students.

• Explain the science behind Tollen’s test.  Tollen's test uses Tollen's reagent that primarily contains silver ions (Ag+). The test works on the basic reduction reaction of these Ag+ ions to an elemental form of silver (Ag) that gives a “silver mirror effect” in the test tube. This is the reason why Tollen’s test is also called the Silver Mirror Test. As the Ag+ ions are reduced, simultaneously, aldehyde is oxidized to carboxylic acid. 

Ag+ ions🡪Ag [Reduction reaction]

Aldehyde🡪Carboxylic acid [Oxidation reaction]

Figure: A basic redox reaction underlies Tollen’s test. Image Source

• Explain the composition of Tollen’s reagent. Tollen’s reagent is composed of silver nitrate, sodium hydroxide and ammonium hydroxide; the chemical formula of the coordinate complex is [Ag(NH3)2]OH. The reagent is colorless and the change to silver color confirms the presence of an aldehyde group in food samples, i.e. reducing sugar’s presence. The reagent is basic due to the presence of OH- ions along with Ag(NH3)2+ ions. 

5. Use virtual lab simulations

Tollen’s test is a practical technique used for the detection of reducing sugars in food-based, fragrance, flavor and various other 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 Tollen’s Test simulation. It takes your students into a virtual world where they can understand how Tollen’s reagent releases silver ions that get oxidized in the presence of an aldose functional group of reducing sugars (both aliphatic and aromatic aldehydes). They can also learn about the chemical structures of reducing and non-reducing sugars and understand why Tollen’s test gives a negative result for non-reducing sugars by engaging with the gamification elements of the simulation. 

As they virtually test for the presence of reducing sugars in different food samples, they can better understand the underlying principle of Tollen’s test. With virtual laboratory simulations from Labster, teachers can make more insightful points as students are rendered with better visual options with multiple games and quizzes to keep the class fun-filled. By 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”. 

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