A third way of illustrating a dynamic equilibrium is by using a paper ball throwing activity. This is a popular activity with students that allows them to collect some quantitative data.

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Activity directions

Make two parallel lines about 2 or 3 metres apart using rope or tape. In step 1, divide students into two equal groups, A and B, standing behind each line. Place 40 balls of screwed-up paper on the floor with one group. Give this group a sign to start throwing the balls, one at a time, at the other group. The students in the other group should throw the balls back again. Only one paper ball can be touched at a time. Every 5 seconds stop the activity and record the number of paper balls on each side. Continue the activity for about 2 minutes. Draw a graph representing the number of balls behind the lines with group A against time.

In step 2 repeat the process, starting with just two students in group A with all the paper balls and the rest of them in group B behind the other line. The number of students throwing the paper balls determines the relative rate of the forward and reverse reactions.

In step 3 repeat the process starting with just two students in group A and the rest of them in group B behind the other line with all the balls.

Understanding the analogy

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Look at the graphs above. Which graph would result from step 1?

Graph B would result from step 1 because equilibrium would be reached with an equal number of paper balls behind each line.

Which graph would result from step 2?

Graph A would result from step 2 because equilibrium would be established with more balls on the side with two students.

Which graph would result from step 3?

Graph C would result from step 3 because equilibrium would be established with more balls on the side with two students. It's the same equilibrium position as that established in step 2.

Like all analogies, care must be taken to identify its strengths and weaknesses in relation to the concept it is used to illustrate, and to prevent the analogy reinforcing student misconceptions.

What misconceptions might this analogy inadvertently reinforce?

The idea that the forward and reverse processes are separate reactions that occur in different compartments.