Pure Water Barely Conducts Electricity

What students typically write

“You can electrolyse water to get hydrogen and oxygen.” Students state this without qualification, implying that you simply pass current through pure water and collect the gases.

Why it is wrong

Electrolysis requires mobile ions to carry the current through the liquid. Pure water is a very poor conductor because it is a covalent molecular substance that barely ionises. The equilibrium H2O reversibly forming H+ and OH- lies overwhelmingly to the left. Only about 1 in every 10 million water molecules is ionised at any given moment. This gives pure water a very high resistance, and negligible current flows.

To electrolyse water, you must add a soluble ionic substance that provides ions for conduction. The standard choice at IGCSE level is dilute sulfuric acid (which provides H+ and SO4 2- ions) or dilute sodium hydroxide. The acid or alkali makes the water conductive without changing the products: hydrogen forms at the cathode and oxygen forms at the anode.

The sulfate ions and the sodium/hydroxide ions are not consumed at the electrodes (under these conditions). They serve only as charge carriers, and the net reaction is the decomposition of water: 2H2O goes to 2H2 + O2.

What the mark scheme actually wants

A typical question: “Describe how you would electrolyse water to obtain hydrogen and oxygen. State why pure water cannot be used on its own.” [3]

Marking points:

1. Add dilute sulfuric acid / an acid or alkali to the water (1 mark)
2. Pure water does not contain enough ions / does not conduct electricity (1 mark)
3. Hydrogen at cathode, oxygen at anode (in 2:1 volume ratio) (1 mark)

Omitting the acid/alkali step typically loses mark 1, and claiming pure water works fine contradicts mark 2.

Worked example: wrong vs right

Question: Explain why dilute sulfuric acid is added to water before electrolysis. [2]

Wrong answer: “The sulfuric acid reacts with the water to make hydrogen and oxygen.” This scores 0/2. The acid does not react with the water to form the products. It provides ions.

Right answer: “Pure water contains very few ions and is a poor conductor (1). Adding dilute sulfuric acid provides H+ and SO4 2- ions, which carry the current through the solution / make it an electrolyte (1).” This scores 2/2.

How to avoid this mistake

Whenever you describe the electrolysis of water, always mention the added electrolyte. The standard exam phrasing is “electrolysis of acidified water” or “electrolysis of water containing dilute sulfuric acid”. This shows the examiner that you understand why the acid is there.

Remember the rule: electrolysis requires an electrolyte, and an electrolyte must contain mobile ions. Pure covalent substances (water, ethanol, hexane) do not qualify. Only ionic substances that are molten or dissolved provide the ions needed.

For the full treatment of electrolysis conditions, see electrolysis. Aqueous electrolysis and electrode product prediction rules are at electrolysis of aqueous solutions.