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Lesson: Chapter 1 — The Chemical Basis of Life

1.1. Carbohydrates 7 of 7

Tests to identify carbohydrates

You cannot tell just by looking whether a food contains starch or sugar. But a few simple laboratory tests make it easy to find out whether starch, monosaccharides or disaccharides are present.

TEST Test for starch

  1. Put a small piece of the food you want to test into a test tube, add a little water, and crush and mix it well.
  2. Add a drop of iodine solution to it.
Identifying starch with iodine solution
Identifying starch with iodine solution

Observation

If starch is present, the mixture turns a dark blue colour tending towards purple (blue-black).

TEST Test for monosaccharides and some disaccharides (Benedict's test)

  1. Put a little glucose solution into a test tube.
  2. Add Benedict's solution little by little.
  3. Stand the tube in a water bath and heat it gently.
Identifying glucose with Benedict's solution
Benedict's test — heating a glucose solution

Observation — the colour change

Blue → Green → Yellow-green → Orange → Brick red

As it heats, the colour changes step by step, and finally a brick red precipitate settles at the bottom of the tube.

TEST Test for a disaccharide (sucrose)

  1. Put a little sugar (sucrose) solution into a test tube.
  2. Add Benedict's solution little by little.
  3. Stand the tube in a water bath and heat it gently. This time no change is seen.
  4. Now add a few drops of dilute sulphuric acid and heat it again.
Benedict's test for sucrose after heating with dilute acid
Test for sucrose — after adding the acid

Observation

Blue → Green → Yellow-green → Orange → Brick red

After the acid is added and the tube is heated, the same colour change seen with glucose now takes place, giving a brick red precipitate.

Why does the result appear only after the acid is added?

A sucrose molecule cannot react with Benedict's solution directly. When it is heated with a dilute acid, sucrose is hydrolysed and splits into its two parts, glucose and fructose. It is these monosaccharides that then react with Benedict's solution.

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