Using the Hofmann Voltameter, we electrolysed water and saw it split into gases — twice as much hydrogen as oxygen. Simple ratio, clear results! We then tested the gases to confirm: hydrogen pops, oxygen relights a glowing splint. Classic electrolysis in action!
Splitting Water with Electricity: A Classic Hofmann Voltameter Experiment
One of the most visually satisfying and memorable experiments in chemistry is the electrolysis of water. Using a piece of apparatus called a Hofmann Voltameter, students can see water being split into its elemental components — hydrogen and oxygen — in real time. It’s a beautiful way to link theory and practice, and it reinforces several key scientific concepts in one go.
What Is a Hofmann Voltameter?
Despite its intimidating name, the Hofmann Voltameter is a simple piece of equipment. It consists of three vertical glass tubes joined at the bottom, forming an H-shape. The outer two tubes collect the gases formed during electrolysis, while the central tube is filled with water mixed with a small amount of sulfuric acid or sodium sulfate to improve conductivity. Electrodes are inserted into the outer tubes and connected to a DC power source.
The Reaction: Water Into Gases
When an electric current is passed through the water:
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At the cathode (negative electrode), hydrogen gas (H₂) forms.
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At the anode (positive electrode), oxygen gas (O₂) forms.
And here’s where the magic happens: you’ll see twice as much gas forming at the hydrogen side compared to the oxygen side. That’s because each water molecule (H₂O) contains two hydrogen atoms for every one oxygen atom. The balanced chemical equation is:
Visual Proof of the 2:1 Ratio
As the experiment runs, bubbles rise in both tubes. The hydrogen side fills much faster — it’s a striking visual representation of the 2:1 hydrogen-to-oxygen ratio in water. You don’t just talk about chemical equations in this lesson — you see them happen.
Testing the Gases
Once you’ve collected enough gas, you can perform the classic gas tests:
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Hydrogen: Hold a lit splint near the mouth of the tube — you’ll hear a squeaky pop, a hallmark of hydrogen igniting.
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Oxygen: Insert a glowing splint into the tube — it will relight, proving the presence of oxygen.
These simple tests are satisfying and safe, and they provide direct evidence of the gases’ identities.
Why This Experiment Matters
This experiment isn’t just a neat trick — it’s a perfect teaching tool for:
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Stoichiometry: Understanding ratios in chemical reactions.
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Electrolysis: Seeing how electricity can cause chemical change.
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Gas tests: Practicing fundamental lab techniques.
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Molecular composition: Reinforcing the H₂O formula with real data.
Tips for Success
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Always add an electrolyte like dilute sulfuric acid or sodium sulfate to help conduct electricity.
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Use a DC power supply (around 6–12 volts).
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Make sure the apparatus is air-tight, or your gas volumes may be inaccurate.
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Collect gases until the volumes are clearly visible and testable.
In Summary
The Hofmann Voltameter offers a powerful demonstration of how water can be split into hydrogen and oxygen. It’s a lesson that combines theory, observation, and hands-on testing — and it never fails to spark curiosity. Whether you’re teaching GCSE Chemistry or A-Level Electrochemistry, this experiment makes an excellent centrepiece for understanding electrolysis in action.
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