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Leaves have special kinds of cells. One type of cell controls what enters and exits the leaf. The plant uses small openings in its leaves (remember that leaves are organs in a plant) to allow gases to enter and leave.

Plant cells use carbon dioxide and the energy of sunlight to make food (photosynthesis). Plant cells must also remove the oxygen (a waste product for the plant) produced in photosynthesis. As plants do photosynthesis, the cells controlling the leaf open to allow oxygen to escape. While it is open, water is also lost from the leaf.

This activity will allow you to determine which side of a leaf is exchanging the most gases by determining which side is losing the most water.

One side of a leaf has more stomata (openings that allow gases to enter and leave the leaf) than the other. Cobalt chloride test paper is blue. When exposed to moisture, the paper turns pink. Using cobalt chloride test paper, find out which side has the most stomata.


  • Four strips of cobalt chloride test paper
  • Transparent tape
  • Plant with leaves
  • Watch, stopwatch, or timer
  1. Go outside and find a plant with leaves still attached and growing on it.
  2. Tape a strip of cobalt chloride paper to each side of 2 leaves.
    • Use one leaf in the sunlight and one leaf in the shade.
    • Remember leaves need to remain attached to the plant!
  3. Every 60 seconds for 10 minutes, record the color of the paper. (Is it still blue, part pink, all pink, etc.?)
  4. Record your data in a table such as found below.
  5. When allowed to sit in a dry location, the cobalt chloride strips will return to their original state and can be reused.

Sample data table:

Time (sec.)

Upper side of leaf in shade

Lower side of leaf in shade

Upper side of leaf in sunlight

Lower side of leaf in sunlight























  1. Which side of the leaf (upper or lower) had a cobalt chloride strip change color the fastest?
  2. What was the difference between the changes in color of the strips which were in sunlight compared to those in the shade?
  3. What will cause a plant to lose less water?
  4. Explain how the specialized cells of a leaf meet the needs of a plant.

Review science safety rules here.

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Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 7th grade science core.

Updated October 24, 2008 by: Glen Westbroek

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