The Fehling’s Test or Benedict’s for use in a Chemistry or a Biology Class. Section 7.4 of chemistry syllabus and of biology syllabus.

Reducing sugars e.g. glucose, fructose, galactose and maltose reduce Benedict’s or Fehling’s solution. Aldehydes also reduce Fehling’s solution. Reducing sugars contain the – CHO functional group. So do aldehydes. It is the – CHO group which makes them good reducing agents.

To do the Benedict’s test follow the method in the next 9 slides. A colour change from blue to red occurs if a reducing sugar or aldehyde is present in the test solution.

To conduct a qualitative test for reducing sugar or an aldehyde

Before you start get or make a copy of this Table of results SampleInitial colourFinal colour A – Glucose solution B – Water

Step 1 Label two boiling tubes A and B.

Step 2 Place 2 ml of glucose solution into tube A.

Step 3 Place 2 ml water into tube B.

Step 4 Add 2 ml of Fehling’s solution into each tube and swirl.

Step 5 Place both tubes in a water bath and heat for 5 minutes.

Expected Result Positive: brick red Negative: blue

Step 6 Table of results SampleInitial colourFinal colour A – Glucose solution B – Water

Some Revision Questions. Name a reducing sugar. Glucose. Name an aldehyde with two carbon atoms per molecule. Ethanal. What reagent is reduced by both aldehydes and monosaccharides. Fehling’s solution.

Some Revision Questions. When doing the Fehling’s test, is heat required? Yes. What is the colour change if the test solution reduces Fehling’s solution? Blue to Red.

Some more Questions. Sucrose, maltose and lactose are three disaccharides. Which is the odd one out? Maltose because it reduces Fehling’s solution or Benedict’s reagent. A compound with three carbon atoms per molecule contains a carbonyl group. It does not reduce Fehling’s solution. Identify this compound. Propanone or acetone.