Test for Reducing Sugars (Benedict's Test)

Categories: ChemistryProteinSugar

Below is the flowchart to reveal the relationship between monosaccharides (simple sugars), disaccharides (complex sugars) and polysaccharides (e.g. starch and glycogen).

Crucial things to keep in mind: (a) Glycosidic bonds are chemical bonds that hold/ join molecules of monosaccharides together.

( b) Chemical solutions of monosaccharides, disaccharides and polysaccharides.

( c) Polysaccharides are macromolecules, implying they are large particles (made up of lots of many little monosaccharide particles joined together in straight or branched chains).

( d) Examples of monosaccharides, disaccharides and polysaccharides.

Test for lowering sugars (Benedict’s Test).

Provided an unknown service, you are to learn if it includes reducing sugars … so you have to carry out the lowering sugar test (Benedict’s test).

What Are Reducing Sugars ??

A lowering sugar (all monosaccharides and some disaccharides) will produce a brick-red ppt when boiles with Benedict’ s solution.

Non-reducing sugar: Sucrose.


  1. To 2 cm 3 of the unknown solution in a test-tube, include an EQUIVALENT VOLUME (that is the very same volume as the unidentified service utilized: 2 cm 3) of Benedict’s service (blue).

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  2. Shake the mix and heat it by immersing the test tube into a boiling water bath (beaker of boiling water) for 5 minutes.
  3. Observe the look of the heated mixture after 5 minutes.


Throughout the heating procedure, the service changes from clear-blue to cludy green, then yellow and finally to a brick-red/ orange-red precipitate (ppt).

Brick-red/ Orange-red/ Red ppt shows the existence of reducing sugars.

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In the Control Expt. above,.

water is used to change the unknown option. Given that water does not contain any decreasing sugars, the mix stays blue after boiling, suggesting the lack of reducing sugars.

Something extra … Benedict’s service is blue due to the existence of copper (II) sulphate.

A decreasing sugar is a sugar with decreasing properties.

When a reducing sugar is warmed with Benedict’s oslution, the blue copper (II) sulphate (soluble) is lowered to form a brick-red ppt of copper (I) oxide (insoluble).

DO NOT OVERHEAT your mixture after the appearance of the red ppt ‘cos it will turn dark brown then black … and these are incorrect color observations!

Test for starch (Iodine Test)


  1. Add a few drops of iodine solution (yellow-brown) onto any substance (liquid / solid).
  2. Observe any color changes.


The iodine solution that stains the substance changes from yellow-brown to blue-black / black , indicating the presence of starch.

In the Control Expt., the iodine solution remains yellow-brown, indicating the absence of starch.


Based on “O” Level Cambridge standards, you can only use the colors blue-black or black


NOT blue to describe the color change.

Saturated Fats vs Unsaturated Fats

2 types of fats:

  1. Animal fats (e.g. butter) are saturated fats.
  2. Vegetable oils and fish oils are unsaturated fats.

Dangers of saturated fats in your diet ….. Saturated fats in the blood can be readily converted into cholesterol .

High level of blood cholesterol would lead to the deposition of cholesterol on the inner walls of blood vessels .

This hardens the blood vessels and decreases the size of lumen (space) of blood vessels , thus decreasing the flow of blood (increasing the blood pressure) through these clogged vessels.

This increases the risk of heart disease (if the blood vessels that supply blood to the heart muscles are clogged).

Dietician recommends … Take less animal fats.

Replace butter with margarine.

Test for fats (Ethanol emulsion test)

Test for fats in liquid food Given an unknown solution, you are to find out if it contains fats …


  1. Place 2 cm 3 of the unknown solution into a dry test tube . Add about 2-5 cm 3 of ethanol (alcohol) into the test-tube.
  2. Shake vigorously for about 1 minute (allowing time for the ethanol to dissolve / extract out the fats).
  3. Pour the mixture into another test-tube containing 2-5 cm 3 of water . Shake the mixture thoroughly.


Upon mixing with water, a cloudy white emulsion is formed, indicating the presence of fats.

In the absence of fats, the mixture should remain clear and colorless.

Test for fats in solid foods Given peanut seeds, you are to find out whether they contain fats


  • Cut the peanut seeds into small pieces and place them into a dry test-tube.

Note: Cutting them into smaller pieces increases the surface area of the food for the extraction of oil by ethanol … and when you transfer the food pieces into the test-tube, please use the forceps and knife (minimise the contact of the food with your fingers ‘cos you do not want to contaminate your specimen!)

  • Add 2-5 cm 3 of ethanol (alcohol) into the test-tube and shake vigorously for 1 minute.
  • Allow the solid food particles to settle at the bottom of the test-tube. Decant (that is pour out only the liquid portion of the mixture, leaving the solid particles behind) the ethanol into another test-tube containing 2-5 cm 3 of water . Shake the mixture thoroughly.


A cloudy white emulsion is formed, indicating the presence of fats.

What is decant? Allow the solid particles to settle down at the bottom of the test-tube, then pour out only the liquid portion.

Know more about proteins

Chemical structure of an amino acid:

Formation of a peptide bond: Condensation reaction between the NH 2 group of one amino acid and the COOH group of another amino acid.

Relationship between amino acid, dipeptide, polypeptide and protein:


A protein molecule is formed by one or more folded polypeptide chain/s.

A short story of how proteins in your food is broken down completely into amino acids and then absorbed into your bloodstream and taken into your body cells to form new proteins …..

Denaturation or “killing” of proteins Folding / coiling of long polypeptide chains will give the protein molecule a distinctive 3-dimemsional configuration / shape .

This distinct shape is important in determining the specific function of the protein molecule.

This distinct shape is maintained by weak hydrogen cross-bonds that keeps the chains folded in a specific manner.

Heat, acids and alkalis break these weak cross-bonds, thus the unfolding of the polypeptide chains and the loss of the distinct shape .

When this happens, the protein can no longer function due to the distortion of its shape, and this is when we say the protein is DENATURED .

Test for proteins (Biuret Test)

Given an unknown solution, you are to find out whether it contains proteins …


  1. To 2 cm 3 of unknown solution in a test-tube, add 1 cm 3 of sodium hydroxide solution . Shake thoroughly.
  2. Add 1% copper sulphate solution (light blue), drop by drop , shaking the mixture after each drop .
  3. Observe color changes.


A violet / lilac color is formed, indicating the presence of proteins.

In the Control Expt.

the mixture remains light blue, indicating the absence of proteins.

Complete proteins VS Incomplete proteins

The proportion of protein in plant food is much lower than that in animal food .

Proteins that come from animals are complete proteins .

What are complete proteins?

Complete proteins are proteins that contain all essential amino acids .

Then … what are essential amino acids?

Essential amino acids are amino acids that the body cannot synthesize/make and must be obtained from our daily diet .

Proteins from plants are incomplete proteins which lack some of the essential amino acids.

For vegetarians to obtain the proper amounts of all essential amino acids, they must eat a mixture of 2 or more incomplete protein foods at each meal.

Sources of complete proteins

Sources of incomplete proteins

Main functions of the 3 major classes of food

Mian function of proteins:

Main function of fats:

Energy values of food

Cite this page

Test for Reducing Sugars (Benedict's Test). (2017, Aug 15). Retrieved from https://studymoose.com/test-for-reducing-sugars-benedicts-test-essay

Test for Reducing Sugars (Benedict's Test)

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