Test for Starch

Aim: To identify the presence of starch in a leaf sample using the Iodine test.

Material required: Fresh green leaves from outdoor plants, beaker, burners, boiling tubes, dropping tube white tile, iodine solution.

Method:

• Boil the leaf in water for about one minute. This kills the protoplasm and bursts the starch grains.

• Remove the leaf and place it in a boiling tube containing about one–third full of 70% alcohol.

• Place in a beaker of water. Warm gently, do not allow the water to boil. The purpose of this stage is to decolourize the leaf.

• Dip the leaf in boiling water again to soften the leaf. Alcohol makes the cell wall brittle.

• Finally, place the leaf on a white tile and drop a few drops of iodine.

Observation:

Based on the observation in the color change of the samples, the following may be concluded as the iodine test for starch results:

• The appearance of blue-black color indicates the presence of the starch in the sample, i.e., a positive iodine test.
• The brown color or no change in the color of the sample indicates the absence of the starch in the given sample, i.e., a negative iodine test.