Why ionic compounds conduct only when molten or dissolved
In the solid state, the ions sit in fixed positions within the giant ionic lattice and cannot move past one another, so the solid does not conduct
In the molten state, the lattice has broken down and the ions are free to move and carry charge through the liquid
In aqueous solution, the water separates the ions from each other and the dissolved ions are again free to move and carry charge
An ionic substance set up so that current can pass through it is called the
Exam tip
Why an ionic compound must be molten or dissolved to conduct
What comes up: "Explain why lead(II) bromide does not conduct electricity in the solid state but does conduct when molten."
Write (two marks): (1) In the solid state the ions are in fixed positions and cannot move. (2) When molten, the ions are free to move and carry charge through the liquid.
Watch out: The mark scheme credits "ions can move" for M2 and explicitly rejects any reference to electrons moving as the reason an ionic compound conducts — ionic conductivity comes from ions, not electrons.
Cations, anions, and the electrodes
A cation is a positively charged ion (Na+, Mg2+, H+)
An anion is a negatively charged ion (Cl−, O2−, SO42−)
In an electrolysis cell, two electrodes dip into the electrolyte and are connected to a d.c. supply
The cathode is the negative electrode
The anode is the positive electrode
Once the supply is switched on, the ions migrate toward the oppositely charged electrode
Cations move to the
Anions move to the
Electrons flow in the external wire from the anode to the cathode, completing the circuit