Principles of Chemistry · 2 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 7% of your exam marks.
Electrode products, half-equations and OILRIG tested in most series.
| Electrolyte | At the cathode | At the anode |
|---|---|---|
| Sodium chloride | Colourless gas, pops with a lighted splint (hydrogen) | Pale yellow-green gas, bleaches damp blue litmus (chlorine) |
| Dilute sulfuric acid | Colourless gas, pops with a lighted splint (hydrogen) | Colourless gas, relights a glowing splint (oxygen) |
| Copper(II) sulfate | Pink-brown solid builds up on the electrode (copper) | Colourless gas, relights a glowing splint (oxygen) |
Describing and identifying products at the electrodes
What comes up: "Describe what you would observe at the cathode / anode" or "Name the gas produced and state the test used to identify it."
Write: Hydrogen — colourless gas; test with a lighted splint and it gives a squeaky pop. Chlorine — pale yellow-green gas; test with damp blue litmus paper and it bleaches the paper (turns it white). Oxygen — colourless gas; test by holding a glowing splint near the opening and it relights. Copper — a pink or pink-brown solid deposit forms on the cathode surface.
Watch out: For hydrogen, the mark scheme rejects "use a glowing splint" — the splint must be lighted (burning), not glowing. For copper, the mark scheme accepts pink, pink-brown, orange-brown, brown, and red-brown as correct colours, but explicitly rejects "red" and also rejects the word "precipitate" — the copper is a solid deposit or coating, not a precipitate.