• During the process of photosynthesis, a plant absorbs, a process which is aided by the presence of chlorophyll in the leaf
  • This enables the plant to convert carbon dioxide and water into glucose
  • Some of this glucose is used up by the plant however
  • Any excess is turned into starch for storage
  • You can, therefore, test a plant’s leaf for starch in order to determine if the plant/leaf is photosynthesising
Testing a leaf for starch. Image credit taskermilward.org.uk

Testing a leaf for starch. Image credit taskermilward.org.uk

ZIMSEC O Level Combined Science: The test for starch

Testing for Starch.

Materials: Iodine solution, dropper, alcohol, test tube, beaker, burner, tripod, gauze, white tile, leaf which was has been in the light.

The test for starch experiment
The test for starch experiment

Test a leaf for starch using iodine solution

  1. Dip the leaf in boiling water

    Put the leaf in a beaker of boiling water. This will kill the leaf and soften it.

  2. Place the leaf in alcohol

    Put the leaf in a test tube of alcohol this will remove its chlorophyll as shown above. Alcohol is highly flammable and should never be heated by a direct flame as it will catch fire. The test tube containing alcohol must be placed in a beaker of boiling water instead.

  3. Remove the leaf from the alcohol and dip it into hot water

    Remove the leaf from the alcohol and dip it into hot water to soften it. You can use a pair of tongs to do this.

  4. Place the leaf on a white tile

    The white tile offers greater colour contrast that will allow you to more easily observe any colour changes

  5. Drop iodine solution onto the leaf

    Using a dropper, cover the leaf with iodine solution.

  6. Observe any colour changes

    Observe the leaf to see if there are any colour changes to the iodine solution you have just placed onto the leaf.

Results

  • If starch is present the leaf will turn blue-black as shown above.
  • If starch is absent the leaf’s colour will remain unchanged.
Results of a positive starch test. Image credit taskermilward.org.uk

To access more topics go to the Combined Science Notes page

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