Aquarium Analogy Illustrating the Concept of Equilibrium

Brief Description: 

Transfer water between clear plastic containers until equilibrium is reached.

Purpose/Goal: 

Equilibrium is reached when the rates of two opposing processes in a system become equal.  The equilibrium is recognized by constancy of macroscopic properties.

Explanation of Experiment: 

Fill one aquarium with water.  If desired, add a few drops of food coloring to make liquid levels more visible.  Leave the other aquarium empty.  If desired, label the full aquarium as "reactants" and the empty aquarium as "products".

Materials Preparation: 
  • Aquaria, equal size, 2
  • Plastic or styrofoam cups, 2
  • Water
  • Food coloring
Presentation: 

Ask for two assistants from the class.  Give each one a plastic cup and the following "rules":

  1. The cups must be as full as possible when transfers are made.
  2. The volunteers must pour at exactly the same time.
  3. Water cannot be caught during the pouring.
  4. The aquaria cannot be tipped.
  5. The cups must be used as scoops, not shovels; the transfers must be done calmly.

Assign one student to transfer from the "reactants" aquarium to the "products" aquarium.  The other student will make the reverse transfer.  As soon as the volunteers are instructed to start, interrupt and ask the class to predict what the water levels in the two aquaria will be when the experiment is finished.  Students will usually predict "half and half", i.e., that the levels will be equal in the two aquaria.  Allow the process to continue without any comments from you.

Hazards: 

Wear chemical splash goggles and always follow laboratory safety rules when performing demonstrations.

Disposal: 

Flush the water down the drain.

Video: 

General Concept:

Primary Reference: 
Bilash, B., Gross, G.R., and Koob, J.K., A Demo A Day, Volume 2 - Another Year of Chemical Demonstrations, Flinn Scientific, Inc., 1998 p. 229