Diffusion in Action

Created Feb. 7, 2024 by Deb Zurenda

This simple demonstration will show diffusion in action within a few minutes. It can be used to illustrate the movement of molecules from a higher concentration to a lower concentration across a permeable membrane.

Materials

  • A solution of iodine (the concentration does not matter)
  • A solution of starch (the concentration does not matter)
  • 2 beakers (A & B)
  • 2 fold-top plastic sandwich bags
  • 2 pieces of string (4 inches in length) 

Procedure

Pour approximately 200ml of the iodine solution into the plastic bag. Twist and tie the top of the bag with the string. Pour approximately 200ml of the starch solution into beaker A. Place the baggie of iodine into the starch solution in beaker A. Repeat by reversing the location of the solutions for beaker B. The iodine will be in the beaker and the starch will be in the plastic bag. 

Results

Within 10 minutes the results of diffusion can be seen, although better results take about 20-30 minutes. Due to the color change (when starch and iodine meet it turns black), it is obvious that the iodine was able to diffuse across the membrane (baggie) but the starch cannot. 

Use in Class

Ask the students to form a hypothesis about beakers A & B. After 20-30 minutes view the results. It may be difficult for students to understand why iodine was able to move but starch could not unless they know about the size of the molecules. Starch is not able to diffuse through the membrane because it is much bigger than iodine.

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