Decomposition of Water


Pre-lab: Answer these questions. Finishing them is your ticket to doing the lab. If you do not have them completed tomorrow, you will finish them in class. You will need your book to answer these questions. Read and use pp 204-207 Answers go on a separate sheet of lined paper.

  1. Use your own words to explain what a reactant is and what a product is
  2. This section talks about balanced chemical reactions. What is being balanced on both sides of any chemical reaction?
  3. What scientific law (from the three below) helps explain why reactions must be balanced?
    1. Heat can be given off during chemical reactions
    2. Energy is needed for chemical changes
    3. You can not destroy matter, but can only change its forms
  1. Explain your answer to question 3.
  2. You and your group will be performing a decomposition reaction. What do you predict will be the products of this reaction?
  3. Fill in the right amounts/numbers in each category


# of atoms?

# of compounds?

# of molecules?











C6H12O6 (sugar)




FeS + HCl



7. The chemical NaOH will be used as a catalyst in this reaction. What is the definition of a catalyst?

8. Sketch the set-up for the "decomposition lab" below














2 test tubes w/ rubber stoppers

2 rubber bands

2 electrodes

power pack & wires


2 burning splints

500 mL beaker


2 ring stands w/ utility clamps


Part A:

  1. Setup apparatus as shown in class. DO NOT PLUG IN POWER PACK (see step 7)
  2. Fill (completely there cannot be any air in the test tubes!) and cork 2 test tubes with water which contains NaOH(aq)
  3. Invert then into a 500 mL beaker containing 250 mL water
  4. Uncork the test tubes with tongs and clamp them to the ring stands
  5. Insert electrodes so that they are partially up into the tubes
  6. Connect electrodes to the power pack
  7. Before the power pack is plugged in and turned on your setup MUST be inspected BY THE TEACHER!!!!

Part B:

  1. Turn off & unplug the apparatus when one of the tubes is empty of most of its water
  2. Disconnect the wires from the electrodes
  3. Remove the electrodes
  4. Carefully disconnect the test tubes from the utility clamps and re-cork them
  5. Take them out of the water and rinse them off thoroughly (to remove the NaOH)
  6. As directed by the teacher - test the test tube that is half full of gas with a glowing splint
  7. As directed by the teacher - test the test tube that is completely full of gas with a burning splint.