- University of Virginia
- Physics Department
Electrolysis of Water
A Physical Science Activity
2003 Virginia SOLs
- observe an endothermic decomposition chemical reaction;
- observe an exothermic synthesis chemical reaction;
- recognize two of the indicators of a chemical change (release of gas and
- analyze the relative densities of the gases;
- analyze the flammability of two gases using glowing and flaming splints;
- gain practice in the following skills: observing, identifying, interpreting
- write out and balance chemical equations for the decomposition and the synthesis
- recognize the use of sulfuric acid as a catalyst and conductor in this investigation.
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.
To print out the Student Copy only, click
- 2 small test tubes, 2 small corks, 2 small clamps
- 1 large test tube, 1 large cork, 1 large clamp
- 2 rubber bands
- 2 j-hook style electrodes
- peg board (the old IPS kind)
- 600 ml beaker half filled with water and a few drops of sulfuric acid
- Two 12-volt batteries
- 3 wires with alligator clips on each end
- 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.
- 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.
- 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
- 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.
- 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.
- 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?
- 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
- 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.
- 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.
- How can you tell which small test tube is collecting hydrogen gas and which
one is collecting oxygen gas before you test them?
- 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
- Why is sulfuric acid added to the water?
- Why does the water start to turn yellow?
- 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
- 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?
- Write a balanced chemical equation for the decomposition of water. Is this
an endothermic or exothermic reaction?
- Write a balanced chemical equation for the synthesis of water. Is this an
endothermic or exothermic reaction?