Electrolysis of Solutions – Splitting Water and Beyond

Electrolysis is one of the most dramatic chemistry experiments. It turns electricity into chemistry by decomposing compounds into their elements. With a Hoffman voltameter, students can see this process in action — as bubbles of hydrogen and oxygen gas appear right before their eyes.

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The Setup

A Hoffman voltameter consists of three vertical glass tubes connected at the bottom and filled with a conductive solution, usually acidified water.

• Electrodes are connected to a direct current power supply.

• When current flows, hydrogen gas forms at the negative electrode (cathode) and oxygen gas at the positive electrode (anode).

• The gases collect in separate graduated tubes, making the reaction measurable and visible.

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The Reaction

$$2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$$

The key observation is the 2:1 ratio of hydrogen to oxygen — twice as much hydrogen gas forms because water contains two hydrogen atoms for every oxygen atom.

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Measuring Results

Students can measure the gas volumes and compare them to the theoretical ratio. Typical results show:

Gas	Electrode	Volume Collected (cm³)
Hydrogen	Cathode	20
Oxygen	Anode	10

When tested, hydrogen burns with a squeaky pop, while oxygen relights a glowing splint — confirming their identities.

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Beyond Water

The same principles apply to other electrolytes:

• Copper(II) sulfate solution: copper forms on the cathode, oxygen at the anode.

• Sodium chloride solution: hydrogen at the cathode, chlorine gas at the anode.

By changing the electrolyte and electrodes, students explore how ion discharge depends on reactivity and concentration.

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Skills Highlight

• Writing ionic equations for electrolysis.

• Identifying products at each electrode.

• Measuring and interpreting experimental gas ratios.

• Linking practical results to energy and bonding.

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Why It Works in Teaching

Electrolysis is one of those experiments that bridges chemistry and physics. The Hoffman voltameter makes an invisible process visible, turning abstract equations into real, testable phenomena. Students don’t just learn what electrolysis means — they see it, measure it, and prove it.