Fehling’s Test: Principle and Procedure Explained

Fehling’s Test is a classic laboratory experiment used to detect reducing sugars and distinguish aldehydes from ketones. Developed by German chemist Hermann von Fehling, this test helps students understand redox (reduction-oxidation) reactions in chemistry through a simple colour change. When a reducing sugar or aldehyde is present, the blue solution turns red due to the formation of copper(I) oxide. 

This article helps students clearly to understand what Fehling’s Test is,its preparations and how it is performed safely, and its importance in both educational and medical chemistry. 

Table of Contents 

• What is Fehling’s Test
• Procedure of the Fehling’s Test
• Lab Precautions to Note: Fehling’s Test
• Common Uses of Fehling’s Test

What is Fehling’s Test ?

Fehling’s Test is a qualitative test that identifies reducing sugars and aldehyde functional groups in organic compounds.

The fact about reducing sugars is that it can donate electrons to other molecules, a property that causes the characteristic color change in this experiment.

What is the principle of the Fehlings Test?

When the test is carried out,  Cu2+ ions from copper(II) sulfate are reduced to Cu₂O, forming a brick-red precipitate. This visual change confirms the presence of a reducing substance.

As Reducing sugars or aldehydes are capable of reducing  Cu2+ to Cu⁺, which then precipitates as red copper(I) oxide.

The general reaction of Fehling’s Test can be represented as:

RCHO+2Cu2++5OH−→RCOO−+Cu2O+3H2O

Here, the aldehyde group (–CHO) is oxidised to a carboxylate ion (–COO⁻), while  Cu2+ is reduced to Cu⁺.

In Fehling’s Test reaction, when tartrate ions are part of the solution, the reaction becomes:

RCHO+2Cu(C4H4O6)22−+5OH−→RCOO−+Cu2O+4C4H4O62−+3H2O

Now, this red Cu₂O precipitate is the clear visual indicator of a positive result of the Fehling's test.

Do you know? The Fehling’s Test is commonly performed in schools because it demonstrates key chemistry concepts such as oxidation-reduction, complex formation, and reactivity of carbonyl compounds.

Next is how Fehling's solution is prepared?

Fehling’s Test uses two separately prepared solutions, Fehling’s A and Fehling’s B, which are mixed just before the experiment.

• Fehling’s A: A blue solution containing copper(II) sulfate CuSO4. It provides the  Cu2+ ions required for the redox reaction.
  
  
• Fehling’s B: A clear solution containing potassium sodium tartrate (Rochelle salt) and sodium hydroxide (NaOH). 

The alkali provides the basic medium, and tartrate ions act as chelating agents to keep copper ions dissolved and prevent precipitation.

What happens is that when equal parts of Fehling’s A and B are mixed, a deep blue complex forms due to the reaction between  Cu2+ and tartrate ions. This freshly mixed solution is Fehling's reagent. 

Procedure of the Fehlings Test 

Here’s how to perform Fehling’s Test step-by-step:

• Place the sample in a dry test tube.
• Use another test tube containing distilled water as a control.
• Add Fehling’s solution to both test tubes.
• Immerse the tubes in a water bath.
• Observe the reaction and record any formation of a red precipitate.

Also Read: Biuret Test 

Lab Precautions to Note: Fehling's Test

• Fehling’s solution is alkaline and corrosive, so wear gloves and goggles.
• Keep a note that always prepares the solution fresh, as it decomposes over time.
• Handle the heating process carefully and do not directly flame the test tube.

Here's the Observation that comes from the test: 

Type of Compound	Reaction with Fehling’s Solution	Observation	Inference
Glucose (Reducing sugar)	Reacts	Brick-red precipitate forms	Positive result
Sucrose (Non-reducing sugar)	No reaction	Solution remains blue	Negative result
Formaldehyde (Aldehyde)	Reacts	Brick-red precipitate forms	Positive result
Acetone (Ketone)	No reaction	No color change	Negative result
Alpha hydroxy ketones	Reacts	Brick-red precipitate forms	Positive result

Common Uses of Fehling’s Test

1. It helps in the detection of reducing sugars,Fehling’s Test helps identify sugars like glucose and fructose that can reduce copper(II) ions.
2.  Aldehydes are oxidised by Fehling’s solution, giving a red precipitate, while ketones remain unaffected which makes the differentiation between aldehydes and ketones.
3. In Medical Applications, earlier, Fehling’s Test was used to detect glucose in urine to screen for diabetes. Though replaced by advanced enzymatic tests today, it laid the foundation for early biochemical testing.
4. It helps students visualise oxidation-reduction reactions, understand functional groups, and connect theory with laboratory practice, making it easier in the education sector.

Learn More: Acid -Base Titration and Benedict's Test

As we learned, the Fehling’s Test remains a simple yet powerful experiment that helps students grasp the fundamentals of redox chemistry and organic compound identification. While newer tests have replaced Fehling’s Test reaction in modern labs, its educational value in understanding chemical reactions continues to make it a favourite among students and teachers alike.