University of Virginia
Physics Department

Electrolysis of Water

A Physical Science Activity

2003 Virginia SOLs



Students will

Motivation for Learning


Background Information

Students should analyze the chemical formula for water with the teacher and discuss the possible products that could result from this in a decomposition reaction. The teacher will need to review the concept of endothermic reactions that require the constant addition of energy to sustain. In this case, electricity will be that energy source.

Now is a good time to remind students that pure water is not a good conductor of electricity. The addition of sulfuric acid to the water (creating an ionic solution) will improve its conductivity and speed up the reaction. Although some of the sulfuric acid does decompose, turning the water yellow, it does not seem to interfere significantly with the expected 2 to 1 ratio of hydrogen to oxygen gas that is collected.

Illustrate on the chalkboard how water is a bipolar molecule. The hydrogen is the positive end and the oxygen the negative end. Illustrate how this will explain the attraction for hydrogen to the negative side of the battery and the oxygen to the positive side. Review the concept of covalent bonds and how difficult they are to break compared to ionic bonds.

Next, review with students the correct way to test for gases in the lab. Hydrogen is lighter than air and makes a barking or squeaking sound when tested with a flaming wooden splint. Since it is lighter than air, the test tube should be held upside down. Oxygen is about the same density as air and should be tested with a glowing splint to see if it bursts back into flame.


Student Activity

To print out the Student Copy only, click here.




    1. Attach the two small clamps onto the peg board. Position them over the 600 ml beaker of water level with the rim of the beaker.
    2. Fill the two small tests tubes with some of the water. Using your thumb or the corks to seal the top of the test tube, invert it under the water. Try not to leave any air bubbles.
    3. Insert the "J" part of the j-hooks under each test tube beneath the water. Secure them at the top with rubber bands. Clamp the test tubes in place.
    4. Lower the j-hooks so that electricity can be conducted through the water and not be impaired by the glass of the test tubes. Be sure the electrodes are positioned so that the bubbles will float up and into the tubes.
    5. Connect your batteries into a series circuit. There should be a complete "circle" of electricity with the water making the connection in the last bit of the circle. Remember that positive poles always connect to negative poles and vice versa.
    6. You should see bubbles forming and floating up into the test tubes at a good rate. Soon you will be able to see that the hydrogen test tube is filling twice as fast as the oxygen test tube. Why is that?
    7. When the hydrogen test tube is full of gas, disconnect the battery and test the gases for flammability: oxygen = right side up, glowing splint; hydrogen = upside down, flaming splint
    8. Repeat the experiment. This time use one large test tube and put both electrodes under it. Be careful not to let the electrodes touch. An explosion under water can be very messy.
    9. When the large test tube is full with a 2 to 1 mixture of hydrogen and oxygen gasses, test their flammability using a flaming splint. Hold the test tube upside down and DONíT LET GO, NO MATTER WHAT HAPPENS!!!!

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.



  1. How can you tell which small test tube is collecting hydrogen gas and which one is collecting oxygen gas before you test them?

  2. Which gas is attracted to the positive and which gas is attracted to the negative electrodes? Can you explain why each of these gasses prefers a certain electrode?

  3. Why is sulfuric acid added to the water?

  4. Why does the water start to turn yellow?

  5. Explain how to correctly test for hydrogen and oxygen gases in the lab. Remember to take into account whether each gas is lighter or heavier than air.

  6. What happened when you tested the flammability of both gases in one test tube? Why was this more violent than the flames produced by either gas alone?

  7. Write a balanced chemical equation for the decomposition of water. Is this an endothermic or exothermic reaction?

  8. Write a balanced chemical equation for the synthesis of water. Is this an endothermic or exothermic reaction?