the fehlings test or benedicts for use in a chemistry or a biology class. section 7.4 of chemistry...

The Fehling’s Test or Benedict’s for use in a Chemistry or a

Biology Class.Section 7.4 of chemistry syllabus

and 1.3.4 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

Sample Initial colour Final 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

Sample Initial colour Final 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.