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Electrolysis of Water

Virtual lab · IGCSE 0620 · GCSE Chemistry
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Hydrogen · cathode
0.00cm³
Oxygen · anode
0.00cm³
Volume ratio
-
Current
0.00A
Charge passed
0C
Controls
1
Prepare the electrolyte
To do
Distilled water - only a trace of ions
A few drops of dilute H₂SO₄ supply the ions that carry the current. The acid is not used up.
2
Power supply
6.0 V DC
Try it with pure water first - watch the ammeter stay at zero.
3
Run the electrolysis
Paused
For revisionTheory, method and exam tips

What this experiment shows

Passing a direct current through acidified water splits it into its elements: hydrogen at the cathode and oxygen at the anode, in a volume ratio of exactly two to one. That ratio is the formula of water made visible. A little acid is added because pure water barely conducts - the acid supplies the free ions the current needs, but is not itself used up.

2H2O → 2H2 + O2

What you need

  • A Hofmann voltameter, or two inverted test tubes over electrodes
  • Distilled water with a little dilute sulfuric acid
  • Inert electrodes - platinum or graphite
  • A d.c. supply, an ammeter and a stopclock

How it is done

  1. Fill the apparatus with acidified water and make sure both tubes are full to the top.
  2. Connect the d.c. supply and note which electrode is which.
  3. Let the current run and watch bubbles form at both electrodes.
  4. Read the volume in each tube at the same moment.
  5. Test each gas: a lit splint at the cathode gas, a glowing splint at the anode gas.

What you should see

  • Twice as much gas collects at the cathode as at the anode.
  • The cathode gas burns with a squeaky pop - hydrogen.
  • The anode gas relights a glowing splint - oxygen.
  • The volumes follow Faraday: the charge passed fixes how much gas is made.

Where marks are lost

  • Getting the electrodes the wrong way round. Reduction happens at the cathode - hydrogen there.
  • Saying the acid is used up. It is not; it only makes the water conduct.
  • Quoting a 1:2 ratio the wrong way round. Hydrogen is the larger volume.
  • Using copper electrodes. Copper is not inert and takes part in the reaction.

Questions students ask

Why add acid to the water?
Pure water contains very few ions, so it hardly conducts. A little acid supplies ions to carry the current without changing the products.
Why twice as much hydrogen?
Each water molecule contains two hydrogen atoms for every oxygen atom, and equal volumes of gas contain equal numbers of molecules, so the volume ratio matches the formula.
How are the gases identified?
Hydrogen burns with a squeaky pop from a lit splint. Oxygen relights a glowing splint.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.