Fehling’s Test Explained | Reducing Sugars Detection (Easy Practical Demo)

Fehling’s Test is an important biochemical test used to detect the presence of reducing sugars such as glucose, lactose, and maltose. This test is widely performed in undergraduate laboratories in biochemistry, microbiology, and life sciences. In this video, you will learn the principle, procedure, results, and interpretation of Fehling’s Test in a clear and practical way. 🔬 What You Will Learn Definition of reducing sugars Principle of Fehling’s Test Composition of Fehling’s solutions Step-by-step procedure Observation and result interpretation Precautions and common mistakes Viva questions 🧪 What is Fehling’s Test? Fehling’s Test is used to detect reducing sugars, which have free aldehyde or ketone groups capable of reducing metal ions. Examples: Glucose Lactose Maltose Non-reducing sugar: Sucrose (negative unless hydrolyzed) ⚗️ Principle of Fehling’s Test The test is based on a redox reaction in alkaline medium. Copper(II) ions (Cu²⁺) → reduced to Copper(I) oxide (Cu₂O) Formation of brick red precipitate indicates positive result 🧫 Composition of Fehling’s Reagent Fehling’s A: Copper sulfate solution Fehling’s B: Sodium potassium tartrate + sodium hydroxide 👉 Both solutions are mixed freshly before use. 🧪 Materials Required Fehling’s A & B Test tubes Pipette Water bath Sample solution 🔬 Procedure Take equal volumes of Fehling’s A and B Mix to prepare reagent Add sample solution Heat in boiling water bath Observe color change 🎨 Observation and Result Blue solution → Negative (no reducing sugar) Brick red precipitate → Positive (reducing sugar present) ⚠️ Precautions Prepare reagent fresh Use clean glassware Do not overheat Handle hot solutions carefully ❌ Common Mistakes Using old reagent Not mixing A and B properly Overheating the solution 🎓 Viva Questions What is Fehling’s Test used for? Why are two solutions used? What indicates a positive result? Why is the reaction alkaline? Can sucrose give a positive result? 🧠 Importance Detection of reducing sugars Used in clinical and food analysis Important for practical exams 📚 Related Tests Benedict’s Test Barfoed’s Test Molisch Test Iodine Test