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Activity: Electrolysis of Water
NGSS Science and Engineering Practices
NGSS Crosscutting Concepts
NGSS Disciplinary Core Ideas


  • Goggles
  • Two 1-mL wide stem transfer pipettes
  • One 1-mL graduated pipette
  • Scissors
  • Ruler
  • One 0.7 mm graphite stick
  • One pin
  • One small paper or plastic cup
  • Baking soda
  • Distilled water
  • 9-V battery
  • Two alligator clip wires
  • Permanent marker


Safety Note: When running your experiment, make sure not to get the battery wet. Use other appropriate safety precautions and be careful.
  1. Cut the cup so it is approximately 3 cm high.
  2. Cut each beral pipette stem 2 cm below the bulb. Each cut pipette should be the same height.
  3. Break the graphite stick so you have at least two pieces that are about 1.5 cm long.
  4. Push the pin into one side of each of the pipette bulbs about 1/3 of the way up the bulb from the stem (Fig. 1.9).
    1. Create the hole in the smooth part of the bulb, not in the seam.
    2. Gently push a piece of graphite into each hole. If the hole is not large enough, you may have to wiggle the pin in the hole to make it bigger.
Image caption

Fig. 1.9. Equipment set-up for electrolysis of water

Image copyright and source

Image by Byron Inouye


  1. Make a saturated solution of baking soda by mixing baking soda into distilled water until it no longer dissolves. Pour the baking soda solution into your small cup, to about 1/3 full.
  2. Fill the cut pipettes completely with the baking soda solution.
    1. Use the graduated pipette to fill the cut pipettes with water.
    2. When filling the pipette bulbs, do not squeeze the area of the bulb with the graphite or the graphite might break.
  3. Place the pipettes stem down in the remaining solution in the small cup. Make sure that the pipette stem holes are completely submerged in the solution (Fig. 1.9).
  4. Attach the alligator clips to the graphite electrodes, being careful, not to break the graphite.
  5. Attach the alligator clips to the battery, one clip on each terminal.
  6. Observe the system as you allow it to run.  Keep in mind the following questions:
    1. Which electrode on the battery is each connected to which electrode on the electrolysis apparatus?
    2. What is happening at each electrode?
  7. Image
    Image caption

    Fig. 1.10. Measure the distance from the bottom of the curve to the top of the solution level on each pipette.

    Image copyright and source

    Image by Byron Inouye

    When you finish running the system, mark the level of the liquid in each pipette with a permanent marker (Fig. 1.10).

    1. You can pull the pipettes out of the cup to do this, just be careful not to squeeze them or mix them up.  
    2. Make a second mark on each pipette where the bulb stops curving at and becomes straight.
    3. Measure the distance between each mark on each pipette and record your results.
  8. Compare your results to the results from the rest of your class. Get a class average for both the pipette bulb with more gas and the pipette bulb with less gas.
Activity Questions
  1. What did you observe as the system ran?
  2. Why was it important that the pipettes were both resting in the baking soda solution?
  3. What happened when the solution level fell below the graphite in one of the pipettes? Why?
  4. What is the chemical formula of water?
  5. Answer the following questions based on the gas formed at each electrode.
    a. How much gas formed at each electrode?
    b. How do the volumes of the gases compare?
    c. How might you explain any differences?
    d. How does this provide evidence for or against the chemical formula for water?
  6. What was the gas generated at the electrode connected to the positive (+) pole of the battery? Give your evidence.
  7. What was the gas generated at the electrode connected to the negative  (–) pole of the battery? Give your evidence.


Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.