In electrolysis, each observation at an electrode comes from one ion gaining or losing electrons. Read the observation, decide the ion, and check the electrode is the right one for that charge.
This lesson follows explaining current flow in an electrolyte and belongs to SPM Science electrochemistry.
How do you turn an observation into an ion?
Work in three short steps. First, note the electrode and the observation. Second, decide whether a positive or negative ion would be discharged there. Third, name the product.
Positive ions go to the cathode and gain electrons. Negative ions go to the anode and lose electrons.
| Observation | Electrode | Ion discharged | Product |
|---|---|---|---|
| Pink-brown solid forms | Cathode | Cu²⁺ | Copper metal |
| Colourless gas that relights a glowing splint | Anode | OH⁻ (from water) | Oxygen |
| Pale green gas with a sharp smell | Anode | Cl⁻ | Chlorine |
| Colourless gas that burns with a pop | Cathode | H⁺ | Hydrogen |
Worked example: three observations from one experiment
In an invented experiment, copper(II) chloride solution is electrolysed with carbon electrodes. The class records three observations.
- A pink-brown coating grows on the electrode connected to the negative terminal.
- Bubbles of a pale green gas form at the other electrode.
- The blue colour of the solution slowly fades.
Observation 1. The negative electrode is the cathode. A pink-brown coating is copper, so Cu²⁺ ions gained electrons: Cu²⁺ + 2e⁻ → Cu.
Observation 2. The other electrode is the anode. A pale green gas is chlorine, so Cl⁻ ions lost electrons: 2Cl⁻ → Cl₂ + 2e⁻.
Observation 3. The blue colour comes from Cu²⁺ ions. As they are discharged, fewer remain in the solution, so the blue fades.
The mistake that costs marks
The slip is to swap the electrodes, writing “copper forms at the anode”. The reasoning sounds plausible, since copper is a metal, but the charge decides the electrode.
| Step | Wrong | Right |
|---|---|---|
| Charge of Cu²⁺ | Not considered | Positive |
| Attracted to | Anode | Cathode |
| Reason | “Metals go to the anode” | Opposite charges attract |
A quick check: metals form positive ions, positive ions need electrons, and electrons arrive at the negative electrode. That is the cathode.
Check yourself
Dilute sodium chloride solution is electrolysed with carbon electrodes. Bubbles form at both electrodes. A gas at the cathode burns with a pop. Name the gas and the ion it came from.
Answer
The gas is hydrogen. It forms at the cathode, so a positive ion was discharged: H⁺ ions from water gained electrons.
The equation is 2H⁺ + 2e⁻ → H₂. Sodium ions stay in solution because hydrogen is discharged in preference to sodium.
What to study next
Next, see how the same chemistry produces electricity in a comparison of simple cell designs. Practise the full topic with the electrochemistry practice set.
If you want a teacher to read your observation answers with you, see online one-to-one Science tuition.