Electrolysis of water is a classic school demonstration that shows how electrical energy can drive a chemical change. With a 9V battery, two electrodes, and water containing a safe electrolyte such as baking soda, students can observe bubbles forming as water molecules split into hydrogen and oxygen gases. The project connects electricity, chemistry, energy transfer, and conservation of matter in one visible setup.
It also helps explain how hydrogen fuel can be produced, though real industrial systems use more controlled and efficient equipment.
Pure water conducts electricity very poorly, so an electrolyte is added to allow ions to carry charge through the liquid. At the cathode, electrons are supplied and hydrogen gas forms, while at the anode, electrons are removed and oxygen gas forms. The overall reaction is 2H2O(l) -> 2H2(g) + O2(g), which means hydrogen is produced in about twice the gas volume of oxygen.
For safety and clean results, baking soda is preferred over table salt because chloride ions from salt can produce chlorine gas at the anode.
Key Facts
- Overall reaction: 2H2O(l) -> 2H2(g) + O2(g).
- Cathode reaction in basic solution: 2H2O + 2e- -> H2 + 2OH-.
- Anode reaction in basic solution: 4OH- -> O2 + 2H2O + 4e-.
- Hydrogen forms at the negative electrode, called the cathode.
- Oxygen forms at the positive electrode, called the anode.
- The gas volume ratio is H2:O2 = 2:1 because each water molecule has two hydrogen atoms and one oxygen atom.
Vocabulary
- Electrolysis
- Electrolysis is the use of electrical energy to cause a nonspontaneous chemical reaction.
- Electrode
- An electrode is a solid conductor that transfers electrons between a circuit and a chemical solution.
- Cathode
- The cathode is the electrode where reduction occurs and hydrogen gas forms during water electrolysis.
- Anode
- The anode is the electrode where oxidation occurs and oxygen gas forms during water electrolysis.
- Electrolyte
- An electrolyte is a dissolved substance that produces ions and allows electric current to pass through a liquid.
Common Mistakes to Avoid
- Using pure water only. Pure water has very few ions, so the current is too small and gas production may be hard to see.
- Using table salt as the electrolyte. Salt contains chloride ions, which can form chlorine gas at the anode instead of only oxygen.
- Reversing the gas labels. Hydrogen forms at the negative cathode and oxygen forms at the positive anode in this demonstration.
- Expecting equal gas amounts in both tubes. The balanced equation shows that hydrogen gas forms at twice the volume of oxygen gas.
Practice Questions
- 1 A student collects 12 mL of hydrogen gas during water electrolysis. What volume of oxygen gas should form if the reaction follows the expected 2:1 ratio?
- 2 If 36 mL of total gas is collected and the gases are in a 2:1 hydrogen to oxygen ratio, how many milliliters of hydrogen and oxygen are present?
- 3 A group switches from baking soda solution to table salt solution and notices a different smell near the anode. Explain why this change is unsafe and how the electrolyte affects the reaction products.