To explain an electrochemical observation, name the particle that moves, say which electrode it goes to, and write what happens to it there. Each observation, such as a coating or bubbles, then has a particle-level cause.
This lesson follows combining an equation ratio with rate data in integrated chemical reasoning. It joins the chemistry of ions with the physics of charge in a circuit, which is why it suits the double-award route.
What is happening at each electrode?
Electrolysis uses direct current to decompose an ionic compound that is molten or dissolved. The ions must be free to move for the circuit to complete.
- Cathode (negative): positive ions arrive and gain electrons (reduction).
- Anode (positive): negative ions arrive and lose electrons (oxidation).
- Wires: electrons flow from the anode to the cathode through the external circuit.
- Electrolyte: ions flow, carrying charge between the electrodes.
For the explanation to score, link each observation to one of these four statements.
Worked example
The observations below are invented for practice. A student passes current through blue copper(II) sulfate solution using inert electrodes, which are electrodes that do not react. This is a teacher-supervised school demonstration, and it is described here only conceptually.
Observations:
- A pink-brown solid appears on the negative electrode.
- Bubbles of a colourless gas form at the positive electrode.
- The blue colour of the solution slowly fades.
Observation 1: Cu²⁺ ions move to the cathode and gain electrons: Cu²⁺ + 2e⁻ → Cu. Solid copper is the pink-brown deposit.
Observation 2: hydroxide ions from water are discharged at the anode, giving oxygen: 4OH⁻ → 2H₂O + O₂ + 4e⁻.
Observation 3: blue colour comes from Cu²⁺ ions. As they turn into copper atoms, fewer remain in the solution, so the colour fades.
A quantitative link: to deposit 0.0100 mol of copper (relative atomic mass Ar = 64), each Cu²⁺ needs 2 electrons, so 0.0200 mol of electrons are needed. The anode reaction releases 4 electrons per O₂, so 0.0200 ÷ 4 = 0.00500 mol of O₂ forms. At 24 dm³ per mole this is 0.00500 × 24 = 0.12 dm³ = 120 cm³. The mass of copper is 0.0100 × 64 = 0.64 g.
The mistake to watch for
Mistaken answer: “Electrons flow through the solution from the anode to the cathode, so the copper ions pick them up.”
Electrons do not travel through the solution. They move through the wire to the cathode, where the copper ions pick them up from the electrode surface. In the liquid, ions carry the current. Correct version: “Cu²⁺ ions move to the cathode and gain two electrons each from the electrode, forming copper atoms.”
Check yourself
All observations below are invented.
1. In molten lead(II) bromide, grey metal forms at one electrode and an orange-brown gas at the other. Which electrode makes the metal, and why?
Show answer
The metal forms at the cathode. Pb²⁺ ions are attracted to the negative electrode and gain two electrons each: Pb²⁺ + 2e⁻ → Pb.
2. Why will solid lead(II) bromide not conduct, but the molten compound does?
Show answer
In the solid, the ions are held in fixed positions and cannot move. When molten, the ions are free to move and carry charge between the electrodes.
3. How many moles of electrons are needed to deposit 0.0300 mol of copper from Cu²⁺?
Show answer
Each Cu²⁺ needs 2 electrons. 0.0300 × 2 = 0.0600 mol of electrons.
Where this leads next
Next comes organic chemistry, in linking organic structure with a stated transformation. When you are ready, work through the mixed practice set. The scientific investigation critic can help you question whether a set of electrode observations supports your explanation.
Students often describe results well but leave out the particle that caused them. An individual teacher in online one-to-one Co-ordinated Sciences tuition can listen to your explanation and ask the follow-up question an examiner would.