Learning Activities: Observing Using Smell
Suggested Grades K-6

Diffusion of a substance through gas molecules
Amphibian absorption of water

20 to 30 minutes

Catalyst Question
What is diffusion?

Diffusion is a basic concept of molecular movement that helps explain the actions of many systems. It is particularly essential for understanding the malformities observed in Minnesota's frogs as it is related to their permeable skin.

Molecules move from an area of higher concentration to an area of lower concentration in a process called diffusion. This process describes the movement of molecules through air, a liquid, or in and out of a cell. Certain materials will either enter or leave cells by diffusion. The direction of diffusion within a cell will depend only on the difference in concentration of water on opposite sides of the cell membrane.

The diffusion of water across a semi-permeable membrane from a region of high concentration of water to a region of low concentration of water is called osmosis. (Semi-permeable membranes are selective for the substances that can pass through them, i.e. molecules of a certain size can pass through, and molecules bigger than this cannot pass through the membrane.) When the molecules reach the point where they are moving in and out at the same rate, this is called equilibrium.

After completion of this activity students should be able to:

1. State what diffusion means
2. Understand the relationship of diffusion to amphibian health

A fragrant substance such as perfume, an orange or a lemon


1. Open a bottle of perfume or squeeze an orange or a lemon
2. Have the students raise their hands when they can smell the fragrance (Students closest to the demonstration should detect the odor first because they are closest to the area where the molecules are bumping into each other.
3. Eventually the molecules move or are diffused throughout the room.

Evaluation/Review Questions
What other examples of diffusion are there?
How does diffusion help explain how a frog's skin absorbs water?
How might the concept of diffusion help explain air pollution?


1. How does the temperature of the surrounding medium affect the rate of diffusion? Try adding a colored substance to warm, tepid, and cold water. In which medium does the substance diffuse the fastest?
2. How does the size of the molecule affect the rate of diffusion? Students should develop an experiment to test how the size of molecules changes the diffusion rate. See the biology website below for ideas.

References and Resources
National Wildlife Federation. 1987. Ranger Rick's Nature Scope: Let's Hear it for Herps. Washington, DC: National Wildlife Federation.

Stebbins, Robert C. and Nathan W. Cohen. 1997. A Natural History of Amphibians. Princeton, NJ: Princeton University Press.

Julvet, Marice. 1993. The Fascinating World of Frogs and Toads. New York: Barens Education Series.

Internet Resources

Education Standards
Click here for more information on how this activity correlates with standards.
6 Science Application (Living Systems)
Content Standard C Life Science (organisms and environments)
Strand 1 Questioning and Analysis Skills
Strand 2.1 The Earth as a Physical System
Strand 2.2 The Living Environment

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