Science — AI-generated practice question

In the electrolysis of water, the two gases collected are hydrogen (H₂) at the cathode (negative electrode) and oxygen (O₂) at the anode (positive electrode).

Their volumes differ because water decomposes according to the equation:

$$2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$$

For every 2 molecules of oxygen released, 4 molecules of hydrogen are produced. Therefore, the volume of hydrogen collected is double the volume of oxygen, since gases at the same temperature and pressure occupy volumes proportional to their molar ratios (2:1).

Source: Activity 1.7, Chapter 1 (Chemical Reactions and Equations)

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• Examiners expect you to name both gases and link them to the correct electrodes (1 mark), state the chemical equation (1 mark), and explain the 2:1 volume ratio from stoichiometry (1 mark).
• A common mistake is confusing which electrode produces which gas — hydrogen forms at the cathode (negative), oxygen at the anode (positive).
• The volume ratio (H₂ : O₂ = 2 : 1) directly follows from the balanced equation — this is the key reasoning the examiner wants to see.