Electrolysis of Water

Splitting water into hydrogen and oxygen gases

Electrochemistry Intermediate (High School) 40 min ~$15.00

Objective

Decompose water into hydrogen and oxygen gases using electrolysis, and verify the 2:1 volume ratio predicted by stoichiometry.

Background

Water electrolysis uses electrical energy to drive the non-spontaneous decomposition of water. At the cathode, water is reduced to hydrogen gas; at the anode, water is oxidized to oxygen gas. A small amount of electrolyte (sulfuric acid or sodium sulfate) is added to increase conductivity.

Safety Warnings

  • Hydrogen gas is flammable — no open flames nearby
  • Do not use excessive voltage (above 12V)
  • Ensure proper ventilation

Required PPE

goggles lab_coat

Materials

  • Distilled water (500 mL)
  • Sodium sulfate (Na₂SO₄) (5 g)
    Electrolyte
  • Universal indicator (5 mL)
    Optional, to show pH changes

Equipment

9V battery or DC power supply Two graphite or platinum electrodes Two inverted graduated cylinders or test tubes Beaker (600 mL) Wires with alligator clips Stand and clamps

Procedure

1

Dissolve 5 g of sodium sulfate in 500 mL of distilled water in the beaker.

3 min
2

Fill two graduated cylinders completely with the solution and invert them over the electrodes in the beaker.

5 min
3

Connect the electrodes to the DC power supply or battery. Note which electrode is connected to positive (anode) and negative (cathode).

3 min
4

Turn on the power supply and observe gas bubbles forming at both electrodes.

2 min
5

Collect gas in the inverted cylinders for 15-20 minutes, recording the volume at each electrode every 5 minutes.

20 min Keep flames away — hydrogen is flammable
6

Compare the volumes of gas collected. The cathode (H₂) should collect twice the volume of the anode (O₂).

3 min
7

Optionally, test the cathode gas with a lit splint (squeaky pop test for H₂) and the anode gas with a glowing splint (relights for O₂).

3 min Use small quantities for gas tests

Expected Results

The cathode should collect approximately twice the volume of gas as the anode, confirming the 2:1 ratio of H₂ to O₂. If universal indicator is added, the solution near the cathode turns blue/purple (basic) and near the anode turns red/orange (acidic).

Cleanup

Disconnect the power supply. Dispose of the solution down the drain. Rinse and dry the electrodes.

Details

Category
Electrochemistry
Difficulty
Intermediate (High School)
Duration
40 min
Est. Cost
$15.00
Steps
7
Materials
3

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Frequently Asked Questions

What is the objective of Electrolysis of Water?
Decompose water into hydrogen and oxygen gases using electrolysis, and verify the 2:1 volume ratio predicted by stoichiometry.
How difficult is Electrolysis of Water?
This experiment is rated as Intermediate (High School). It takes approximately 40 minutes to complete.
What safety precautions are needed for Electrolysis of Water?
Key safety precautions include: Hydrogen gas is flammable — no open flames nearby; Do not use excessive voltage (above 12V); Ensure proper ventilation.
What materials are needed for Electrolysis of Water?
The main materials required are: Distilled water, Sodium sulfate (Na₂SO₄), Universal indicator.
What results should I expect from Electrolysis of Water?
The cathode should collect approximately twice the volume of gas as the anode, confirming the 2:1 ratio of H₂ to O₂. If universal indicator is added, the solution near the cathode turns blue/purple (basic) and near the anode turns red/orange (acidic).