Learning Activities: Observing Using Sight
Suggested Grades K-6

Concepts
Diffusion of a substance through a liquid
Amphibian absorption of water
The relationship between equilibrium and diffusion

Time
20 to 30 minutes

Catalyst Question
How does diffusion occur in a liquid medium?

Background
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 a process that occurs through their permeable skin.

Molecules move from an area in which they are highly concentrated to an area in which they are less concentration in a process called diffusion. This process describes the movement of molecules through air, a liquid, or in and out of a cell. It continues until the system reaches a state of equilibrium in which the molecules are randomly distributed throughout the whole system.

Static equilibrium exists when no action occurs.

Example: A Styrofoam cup full of water would be in static equilibrium because the water level stays the same. If the cup had a hole at the bottom water would leak out and the system would not be in equilibrium at all.

Dynamic equilibrium exists when two opposing actions occur at the same rate.

Example: If someone was adding to the cup above at the same rate that it was draining out the bottom the system it would be in dynamic equilibrium. The water level isn't changing but the actual water in the cup is changing. Diffusion occurs when a system is not in equilibrium.

Objectives
After completion of this activity students should be able to:

1. Describe how diffusion occurs within a liquid
2. Explain a situation where diffusion through a liquid might occur in the natural world

Materials
A beaker or glass container of water
Red ink
A dropper

Procedure
1. Tell the students that you are going to drop red ink into a beaker of water. Ask the students to predict what will happen.

2. Drop the ink into the water. At first, the molecules of ink are bunched together. But eventually the water molecules and the ink molecules bump into each other and become mixed. The water turns red.

3. Have the students describe or draw the way that the ink molecules move through the water. At first most molecules move in straight lines away from the center of the drop. They change direction only when they collide with each other or the water molecules. When they reach the glass wall they begin moving back toward the center and past it toward the other side. Diffusion stops when the molecules are evenly spread throughout the water.

4. Explain that the system is now in equilibrium and diffusion no longer occurs.

Evaluation/Review Questions
How might diffusion help explain water pollution?

What factors might influence the rate of diffusion?

Is the system now in equilibrium?

Extensions
Several factors affect the rate of diffusion. Kinetic energy is often measured as the temperature of the system. Molecules in a higher temperature system will diffuse more quickly because they have more energy and move more quickly.

The size of the molecule is a second factor. A smaller molecule will diffuse more rapidly because it takes more energy to get a larger molecule moving.

Finally, any positive or negative charges on the molecule or the material through which it is moving affects the rate of diffusion.

How could you test these different factors? Be careful to change only one variable in each series of tests to make sure your results are scientifically valid!

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

Chinery, M. 1991. Life Story:Frog. Mahway, NJ: Troll Publications.

National Wildlife Federation. 1987. Ranger Rick's Nature Scope: Let's Hear It for Herps. Washington, DC: National Wildlife Federation.

Internet Resources
http://biology.arizona.edu/sciconn/lessons/mccandless/default.html
http://cgee.hamline.edu/frogs
http://tqd.advanced.org/3542/experiments/osmosis.html

Education Standards
Click here for more information on how this activity correlates with standards.
Minnesota
6 Science Application (Living Systems)
National
Content Standard C Life Science (organisms and environments)
Environmental
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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