Introduction: The concept of equilibrium is a very important one to scientists in all fields. Static equilibrium refers to a condition in which the parts of a system have stopped moving, and is rare in nature. Dynamic equilibrium refers to a condition in which the parts of a system are in continuous motion, but they move in opposing directions at equal rates so that the system as a whole does not change. In this exercise, you will explore the concept of dynamic equilibrium.

Materials: Clear plastic containers (2), paper cups, water. meter stick.

Methods: Pour water into the plastic containers until the water level is the same in each container. Measure and record the depth of the water. Obtain two paper cups and submerge each in a separate container of water. When given a signal by one of the lab team members, simultaneously pour each cup of water into the other container. Repeat this process four more times, then measure and record the depth of the water. Then repeat the cycle of five transfers two more times, and measure the depth of the water after each cycle. Record your observations.

Pour all of the water into one of the containers. Measure and record the depth of water in that container. Proceed as before. At the beginning, one cup will be empty and the other full. Simulate the pouring with the empty cup at each transfer. As water is put into the empty container, gather as much water as possible in the cup for each transfer. Measure and record the depth of the water after each five transfers, and continue until twenty transfers have been made. If time allows, do the whole procedure again with different starting depths. Dispose of the water and return the materials to the front desk.

Laboratory Report

Observations:

Prepare a data table to record the changes in the depth of the water in both containers under each set of conditions. The table should have five columns: Time, Trial 1 Container 1, Trial 1 Container 2, Trial 2 Empty Container, Trial 2 Full Container. You may add more columns if you collect more data.

Prepare graphs of your data. Put time on the horizontal axis and depth on the vertical axis. Be sure to indicate the units of measure. Plot both the data for both containers for each trail on the same graph (remember to use different line types).

Conclusions:

1. What happened to the water levels in Trials 1 and 2?
   
   
2. How would you describe the shape of the graph for each set of data?
   
   
3. In Trial 2, one of the cups initially has no water in it. What happens to the amount of water transferred from the empty side to the full side as the trial continues?
   
   
4. Are the final water levels in each trial the same? Why?
   
   
5. What would happen to the water level in each system if you continued to transfer the water indefinitely? Why?
   

Teacher Materials:

Teaching Tips

Evaluation Keys

Student Materials:

	Background Reading
Osmosis in Elodea	Dynamic Equilibrium	Factors Affecting Diffusion
Osmosis	Osmosis and Blood Cells	Quiz

Science Education Connection Department of Biochemistry The University of Arizona Wednesday, February 12, 1997 johnmcc@dakotacom.net http://biology.arizona.edu/sciconn/lessons/mccandless/ All contents copyright © 1997; 1998. All rights reserved.