Activity 8: Equilibrium Experiment

Cameron Weiner - Undergraduate - Middlebury College,

Lisa Gilbert -Professor of Geosciences and Marine Science - Williams-Mystic,

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Students explore the systems thinking concepts of equilibrium and nonequilibrium with a water pouring experiment. Students complete the activity at home or virtually with videos. Water is poured from a top container (reservoir) into a central cup (reservoir) with a small hole for water outflow at a different rate in each trial. Students observe what happens to the water level in the central container, graph the change in water level over time, and assess whether the central reservoir was in equilibrium or nonequilibrium for each trial.

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This equilibrium experiment is designed for a middle school science course. Materials presented here are designed to be implemented in a remote learning environment, either as part of an entirely online or hybrid course.

Skills and concepts that students must have mastered

Students need background information on systems thinking vocabulary (flows and reservoirs) to complete all of the analysis questions. See Activity 1: Introduction to Systems Thinking for an idea of how to introduce systems thinking vocabulary.

How the activity is situated in the course

This activity is the 8th activity in a Systems Thinking module designed for middle school science courses but can act as a stand-alone exercise. Within the systems thinking module, this activity introduces the concepts of equilibrium and nonequilibrium which help students understand how systems and reservoirs can change over time. This activity is designed to be an introduction to Activity 9 which applies equilibrium and nonequilibrium to an earth science system and Activities 10 and 11 which introduce and apply feedback loops to earth science systems. The activity fits well alongside topics of water conservation and scarcity as well as units on lakes, rivers and oceans.


Content/concepts goals for this activity

  • Students will perform or view an equilibrium experiment.
  • Students will graph the change in water level of the central reservoir over time.
  • Students will learn the definition of equilibrium and nonequilibrium.
  • Students will apply the terms equilibrium and nonequilibrium to each trial they performed in the experiment

Higher order thinking skills goals for this activity

Other skills goals for this activity

Description and Teaching Materials



Activity Description (total time: 45-70 min)

  • 5-20 min pre-experiment, 45 min in class, 5 min for optional local adaptation)

Part 1 - Pre-Experiment Instructions and Preparation(5-20 min. pending experiment option)

Prior to the experiment the students should read the Pre-Experiment Instructions Activity 8. These instructions ask students to decide whether they will perform the equilibrium activity in their homes or with videos virtually. 

If students decide to perform the activity in their homes, they will need to prepare the materials and have them accessible at the start of this activity. See the Pre-Experiment Instructions for more information and a set-up video on the materials and set-up the students will need. It would be helpful to review the set-up instructions with students deciding to perform this experiment in their homes.

If the students decide to complete the experiment virtually, they will not need to collect any additional materials before class, but should still review the pre-experiment instructions prior to class.

Part 2 - Engage students with a prediction and introduce the activity(5 min)

The instructor begins screen sharing the Equilibrium Experiment Powerpoint.

Slide 1: The instructor prompts students to think about why sinks might overflow.

The Instructor might prompt students by saying: "Have you used a sink that overflowed or filled up with water but you stopped it before it overflowed? Why might that have happened? If you haven't had that experience think about why it might happen. 

Slide 2: The instructor asks students to think about why sinks might overflow for 1 minute. After 1 minute, the students are sent to breakout groups to discuss their thoughts with a partner for 2 minutes.

Slide 3: The instructor introduces the activity topic: Equilibrium in Systems - Experiment Day! 

Slide 4: The instructor introduces the activity goals: Perform the Equilibrium Experiment and Assess the equilibrium of a reservoir. The instructor might introduce the goals by saying: "Today we will be performing an equilibrium experiment, either in our homes or using pre-recorded videos. You all should have read the Pre-Experiment Handout so you have a basic understanding of what we'll be doing. After we perform the experiment and record our data we are going to graph how the water level changed in our center cup while we were pouring water into that cup. Then we will talk about the definitions of equilibrium and nonequilibrium and decide which of the trials represented a reservoir in equilibrium and which of the trials represented a reservoir in nonequilibrium."

Slide 5: The instructor indicates that the first thing students will do is perform the equilibrium experiment.

Part 3 - Equilibrium Experiment(30 min - 5 for explanation, 25 to complete the experiment)

Slide 6: The instructor opens the Experiment Student Handout Activity 8 and the Data Sheet on google slides. The instructor explains that students who have the materials ready and would like to perform the experiment at home should follow Procedure 1 for each trial. These students should have three containers, one with a premade hole, already assembled. Students will be divided into pairs to complete the experiment. Each student should perform each trial once. The student performing the experiment or playing the video should observe what happens to the water level of the central cup while water is being poured from the top container. The other student should time how long it takes for the other student or the video to pour all the water from the top container into the central container. The students share and record their times and observations. The instructor shows students where and how to record their times and observations in the text boxes on Slide 2 of the Data Sheet

Slide 7: The instructor returns to the powerpoint and asks students for clarifying questions. 

Slide 8: Once the questions are answered, the instructor asks students to open their own Experiment Student Handout Activity 8 and google slides Data Sheet, then tells students they will have 25 minutes to complete the activity. At the 5 minute warning students should try to perform Trial 3 at least one time if they are running behind. Students are sent into breakout group pairs to complete the experiment. 

--After 20 minutes--

The instructor should send out a 5 minute warning to the breakout groups, instructing students to each complete Trial 3 at least once in the last 5 minutes.

--After 25 minutes--

The instructor brings all students back together in one group and screen shares Slide 9 of the Equilibrium Experiment Powerpoint. The instructor asks students to complete the analysis section of their student handout which includes graphing the water level change over time for each trial on slides 3-5 of the google slides Data Sheet and listing the flows and reservoirs of their experiment system. (5 min)

See Instructor notes on safety and an alternative virtual equilibrium experiment.

Part 4 - Equilibrium Explanation and Discussion Questions (10 min)

Slide 10: The instructor prompts students to reflect on their initial prediction: Why do sinks overflow? Has your answer changed? Provide time for a few students to call out their thoughts. Students should realize that sinks can overflow from too much inflow and not enough outflow.

Slide 11: The instructor indicates that they will now move onto the second activity goal: Define equilibrium and nonequilibrium.

Slide 12-15: The instructor defines equilibrium as when the quantity of things in a reservoir or system stays about the same over time, which can also be thought of as when the reservoir or system is considered stable or balanced over time. Nonequilibrium is defined as the opposite: when the quantity of things in a reservoir or system changes over time, which can also be thought of as when the reservoir or system is considered unstable or unbalanced over time. 

Slide 16: The instructor tells students that the activity will now address the final goal: Describe each trial as in Equilibrium or Nonequilibrium. The instructor asks students to complete Part C, the Discussion section on their Experiment Student Handout Activity 8 which asks students to determine if the central reservoir was in equilibrium or nonequilibrium for trials 1-3.

Part 5 - Optional Local Adaptation (5 min)

Slides 17 -18: These final two slides offer an example of how you could tie the experiment into local, water use context. Slide 17 shows the inflows and outflows of the Scituate Reservoir. Slide 18 shows a graph of the water level of the Scituate Reservoir between July 2014 and June 2015. The added straight blue line represents an estimated average yearly water level, while the red line represents the actual water level for each month between July 2015 and June 2015. You could have a class discussion about whether the drinking water reservoir that supplies much of Rhode Island is in equilibrium or nonequilibrium. Over a month timespan the reservoir might be considered in nonequilibrium, increasing in summer months and decreasing in winter months. But because the reservoir generally follows the same trend of decreasing in the winter and increasing in the summer to about the same level (shown by the historical average), the reservoir could be considered in equilibrium. The average inflows and outflows for 2014-2015 were fairly close in magnitude.

Search your water utility company's website or give them a call to see if you get access to reservoir water level graphs or average inflow and outflow data. 

Teaching Notes and Tips

Safety Warning: The Pre-Experiment Handout instructs students who choose to complete the experiment in their homes, and not use the videos, to use scissors to cut their own holes in plastic bottles.

Another option for a virtual adaptation is to have students perform trials 1-3 with Stella Models instead of the videos. These 'bathtub models' were adapted from the undergraduate Systems Thinking module on InTeGrate. The students can run the models themselves by pressing the play button at the bottom left-hand corner of the screen. Students can record the change in water level on similar graphs to those provided in the Google slides Data Sheet, identify the two flows and one reservoir and then complete the same discussion questions.

Model 1: faucet inflow = drain outflow from the bathtub

Model 2: faucet inflow < drain outflow from the bathtub

Model 3: faucet inflow > drain outflow from the bathtub


References and Resources

This systems thinking module is based on the undergraduate Systems Thinking module on InTeGrate, created by Lisa A. Gilbert, Deborah S. Gross & Karl J. Kreutz. This equilibrium experiment relates to Unit 3: Modeling a System.

Experiment Data Table Activity 8

Systems Thinking Vocabulary Glossary

Why teach systems thinking in Middle School?

"Appendix G - Crosscutting Concepts." 2013. Next Generation Science Standards.

Learn about why we should teach Systems Thinking in Earth Science:

Learn more about teaching systems thinking: 

Learn more about systems thinking:

  • Meadows, Donella H., and Diana Wright. 2008. Thinking in Systems: A Primer. White River Junction, Vt: Chelsea Green Pub.