In a metal wire, electrons carry the current. In an electrolyte, free-moving ions carry it. The circuit works only when both parts connect at the electrodes.
This is the first lesson in SPM Science electrochemistry. The next lesson, interpreting electrolysis observations, builds directly on it.
What carries the charge in each part of the circuit?
Think of a relay race. Electrons run along the wire. At an electrode they hand the charge to ions, which carry it across the liquid.
| Part of circuit | Charge carrier | Why it can move |
|---|---|---|
| Metal wire | Electrons | Free electrons in the metal |
| Electrolyte | Ions | Dissolved or molten, so ions are free |
| Solid ionic compound | None moving | Ions fixed in a lattice |
| Sugar solution | None | Molecules, no ions |
Worked example: two beakers, one bulb
An invented circuit has a battery, a bulb and two carbon rods. In test 1 the rods dip in sodium chloride solution, and the bulb lights. In test 2 the rods dip in sugar solution of the same volume, and the bulb stays off.
Test 1. Sodium chloride dissolves into Na⁺ and Cl⁻ ions. Na⁺ ions move to the negative rod and Cl⁻ ions move to the positive rod, so a complete circuit forms and the bulb lights.
Test 2. Sugar dissolves as whole molecules with no charge. Nothing in the liquid can carry charge, so the circuit is broken at the solution and the bulb stays off.
Notice that the water is the same in both. The difference is the presence of ions.
A frame for a full-mark answer
Conduction questions fit this frame: “The substance contains (or forms) ions. The ions are free to move when (dissolved or molten). Positive ions move to the cathode and negative ions move to the anode, carrying charge through the liquid.”
If a question asks about a solid, change the second sentence: “The ions are held in fixed positions so they cannot move, so the solid does not conduct.”
The mistake that costs marks
The common slip is “electrons flow through the solution”. It sounds right, but the carriers in the liquid are ions.
| Statement | Mark? | Reason |
|---|---|---|
| Electrons move through the solution | No | Ions carry charge in the liquid |
| Free-moving ions carry the current | Yes | Names carrier and condition |
| The solution is wet so it conducts | No | Wetness is not the cause |
Check yourself
Solid lead(II) bromide does not conduct, but molten lead(II) bromide does. Explain the difference.
Answer
Lead(II) bromide is an ionic compound. In the solid, the ions are held in fixed positions in the lattice, so no charge carriers can move.
When it is melted, the ions become free to move. The Pb²⁺ ions move to the cathode and the Br⁻ ions move to the anode, so the molten compound conducts.
What to study next
Go on to interpreting electrolysis observations, where the ions you just met are discharged at the electrodes. You can test the whole topic later with the electrochemistry practice set.
If you want a teacher to go through conduction questions with you, see online one-to-one Science tuition.