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Biological Energy - Prove Plant Materials Are In It!

In 7th Grade Integrated Science, you learned that plant cells have chloroplasts in them. Recall that these green plant cell organelles take sunlight and change it into energy for the plant. The process of converting sunlight energy into sugars is called photosynthesis. After the sugar is made, plants combine sugar molecules to make starch.

This activity is designed to help you detect starch in a solution using a chemical indicator. The chemical you will use in the activity is iodine. A solution of iodine will change color in the presence of starch. Iodine can also stain your skin so be VERY careful to follow all instructions!

Materials:

  • Three beakers
  • Water
  • Cornstarch
  • Sugar
  • Balance scale
  • Eyedropper
  • Stirring implement
  • Iodine solution

Procedure:

  1. Fill the beaker with 200 ml. water.
    • Add one drop of iodine solution.
    • Stir the mixture.
    • Observe any reactions
  2. Fill a second beaker with 200 ml. water.
    • Measure 5 g. cornstarch.
    • Stir the cornstarch into the water
    • Add one drop of iodine solution.
    • Stir the mixture.
    • Observe any reactions.
  3. Fill the third beaker with 200 ml. water.
    • Measure 5 g. sugar.
    • Stir the sugar into the water.
    • Add one drop of iodine solution.
    • Stir the mixture.
    • Observe any reactions.

Note: Iodine solution will turn from a golden brown color to a blue-purple color in the presence of starch.

  

  

Safety concerns: Be sure to follow all glassware, eye, and chemical safety rules that are specified by your teacher in all general laboratory experiences. DO NOT allow your beaker to be covered or stoppered. Make sure that you protect your body and clothing from the reactions. Remember that iodine is poisonous and care should be taken to keep away from eyes and mouth. Iodine stains clothing and skin. As with all science lab activities, the most important safety rule is to follow all teacher directions.

Extensions:
You may want to test other substances for the presence of starch. You might try foods such as: bread, crackers, potato, soda, milk, etc.

Assessment:

  1. How do the results of the cornstarch, water and sugar tests differ?
  2. How does iodine signal the presence of starch?
  3. For each test, was the result positive or negative? When testing for a substance, a positive result signals that the substance is present; a negative result means it is not present.
utah state board of education This Sci-ber Text was developed by the Utah State Board of Education and Utah educators.