Introduce students to chemical composition and balanced chemical equations. Easy, safe, and cheap set up and running of the experiment. Works well and students can make observations within a normal lab period

1. Electrolysis of Water
Introduction:
The decomposition of a compound into simpler substances by means of an electric current is called
electrolysis. During the electrolysis of water, electricity is passed through water, which then separates into its
elemental components of oxygen and hydrogen.
Since pure water does not conduct electricity well enough to allow electrolysis to occur a reasonable rate, a
catalyst is needed to speed up the reaction. In this experiment, sodium carbonate in solution will be used to
increase the rate of the reaction. The sodium carbonate ionizes in water, much like NaCl, making the solution
an electrolyte.
Pre-Lab Questions:
1. What are the molecular formulas for Water, Hydrogen, and Oxygen?
2. Looking at the above formulas, how much Hydrogen gas would you expect to be formed from the
electrolysis of water?
3. Looking at the above formulas, how much Oxygen gas would you expect to be formed from the
electrolysis of water?
Procedure:
□ 1. Fill a 250 mL beaker with the provided solution until the height (approximately 200 mL) is above the
ends of the electrodes.
□ 2. Connect your test tubes together with tape. Mark one Test Tube “+” and the other “-“
□ 3. Fill both test tubes with the solution of sodium carbonate and cover them with paper, as demonstrated
by Mrs. Graham
□ 4. Turn them over into the beaker so that the papered end is below the level of the liquid.
□ 5. Remove the paper allowing the test tubes. IF THERE ARE AIR BUBBLES THEN YOU MUST TRY
AGAIN!!!
□ 6. Place your battery in the solution
□ 7. Place the test tubes over the electrode of the battery, making sure to have the “+” test tube over the +
side of the battery and the “-“ test tube over the “-“ side of the battery. Lean the edges of the test tubes
against the side of the beaker.
□ 8. Observe the apparatus and answer some of the lab questions until one of the tubes completely fills
with gas.
□ 9. Remove the test tubes from the battery – BE CAREFUL TO LEAVE THEM UPSIDE DOWN IN
THE SOLUTION!
□ 10. Remove your battery and dry it with a paper towel
□ 11. Testing the gas: Working with your partner, test the smaller quantity of gas with a glowing wood
splint or toothpick. KEEP THE TEST TUBE POINTED DOWNWARD. Lift it up above the water,
and insert the glowing wood splint. Caution: Point the opening of the test tube away from yourself,
your partner, and everyone else when conducting this test.

2. □ 12. Test the larger quantity of gas by placing a flaming toothpick to the opening immediately after lifting
from the water. Reason: If you do not do this immediately, the gas will escape and no reaction will be
visible. Caution: Point the opening of the test tube away from yourself, your partner, and everyone
else when conducting this test.
Questions: - Answer the questions on a separate sheet of paper
1. Compare the volumes of gases obtained from the two sides of the battery. From your answers to the
pre-lab questions, can you figure out which side corresponds to which gas? .
How much of the test tube is
Electrode Identity of the gas
filled? ½, ¾, completely full
Positive
Negative
2. Which gas forms at the positive lead?
Why do you think that this gas forms at the positive contact? (You will have to read ahead
and / or go online to answer)
3. Which gas forms at the negative lead?
Why do you think that this gas forms at the positive contact? (You will have to read ahead
and / or go online to answer)
4. Make a comment about the volumes of gas formed and the formula of water. How would these
observations lead a scientist, like James Dalton, to conclude that compounds are formed of set ratios of
atoms of different elements?
5. (Challenge Question) From your observations, write the balanced equation for the electrolysis of water.
To do this, write the formula for water followed by an arrow and then the formulas for hydrogen and
oxygen. Use the volumes that you observed of the produced gases to decide what numbers you should
put in front of each formula.
For example, a Balanced chemical equation can look like this: A2B3  2A + 3B… which means
that there are 2 A’s formed for every B.
6. Charged particles were needed to make the reaction faster. What ions were present in the solution due
to the sodium carbonate?