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Electrolysis of Water

Summary:  Sodium sulfate solution is electrolyzed to make hydrogen and oxygen gas.  A ignited wood stick shown to glow more brightly in the presence of concentrated oxygen gas (collected in a test tube).  Hydrogen gas collected in a test tube is ignited using a bunsen burner. 


Chemicals and Solutions: 

1.0 M Sodium sulfate solution


Electrolysis Apparatus

D.C. Power Supply

Dilute (0.5M or so) Na2SO4 solution (500 mL minimum to fill apparatus). Phenolphthalein may be added to show formation of hydroxide ion at the cathode.



Test tubes


Open stopcocks, making sure that the sodium sulfate solution filles each compartment to the tip. Close stopcocks. Attach apparatus to D.C. power supply.

Do a check to make sure the demo works.

As soon as class starts, turn on the power supply. Operate at about 10-12V.

Hydrogen may be collect at the cathode in a small test tube and ignited. Oxygen may be collected at the anode.  Add a glowing splint to show that oxygen supports combustion.

Rinse apparatus immediately after use to avoid corrosion of the electrode.

Hint:  Too much voltage will blow the Variac fuse, best to keep demo under ….

Sodium sulfate solution will eventually mould.


The overall reaction:

2H2O(l) ----> 2 H2(g)+ O2(g)

Reduction at the cathode:

Oxidation at the anode:

By multiplying the equation for the reaction at the cathode by two and then combining it to the equation for the reaction at the anode you get:

This equation can be simplified, because the H+ and the OH- combine to give water.  You can then subtract 4H2O from each side to get:

To schedule a demonstration, please send an e-mail to the demonstration lab.



Hakme (hahng-mee) Lee

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606


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