2.1 Testing for Biological Molecules

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe and carry out the Benedict's test for reducing sugars, the iodine test for starch, the emulsion test for lipids, and the biuret test for proteins.
  2. Describe and carry out a semi-quantitative Benedict's test on a reducing sugar solution by standardising the test and using the results (time to first colour change or comparison to colour standards) to estimate the concentration.
  3. Describe and carry out a test to identify the presence of non-reducing sugars, using acid hydrolysis and Benedict's solution.

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Part 1: Qualitative Food Tests

A qualitative test tells you whether a substance is present or not — it gives a yes/no answer. It does not tell you how much of the substance is there.

The four qualitative tests you need to know are:

  • Benedict's test → for reducing sugars
  • Iodine test → for starch
  • Emulsion test → for lipids
  • Biuret test → for proteins

1A — The Benedict's Test for Reducing Sugars

A reducing sugar is a sugar that can donate electrons to another substance (in this case, to copper ions in Benedict's reagent). All monosaccharides (single-unit sugars like glucose and fructose) are reducing sugars. Some disaccharides (two-unit sugars), such as maltose, are also reducing sugars.

What is Benedict's reagent? Benedict's reagent is a blue liquid. It is blue because it contains copper(II) sulfate, which has copper(II) ions (Cu²⁺). These copper(II) ions are what react with reducing sugars.

What happens during the test? When a reducing sugar is present and the mixture is heated, the reducing sugar donates electrons to the copper(II) ions. This changes them into copper(I) oxide (Cu₂O), which is an insoluble solid — meaning it does not dissolve in water. This solid appears as a coloured precipitate (a solid that forms inside a liquid).

Step-by-step procedure:

  1. Place your sample solution into a test tube.
  2. Add an excess of Benedict's reagent to the sample. Using excess means there is more than enough reagent to react with all the sugar present.
  3. Place the test tube in a water bath that has been brought to the boil (100°C). Heat for about 5 minutes.
  4. Observe any colour change.

Results:

Colour of PrecipitateInterpretation
Blue (no change)No reducing sugar present — negative result
GreenVery small amount of reducing sugar
Yellow/OrangeModerate amount of reducing sugar
Brick-red/BrownLarge amount of reducing sugar — positive result

The colour changes progressively: blue → green → yellow → orange → brick-red, depending on how much reducing sugar is present.

Safety precautions:

  • Handle hot test tubes with tongs.
  • Wear safety goggles and heat-proof gloves.
  • Use a water bath rather than a direct flame to heat evenly and safely.

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