Why Ionic Compounds Conduct Electricity

This question appears almost every exam series across Papers 2 and 4 and is worth 2-4 marks. The answer rests on one idea: ions must be free to move to carry charge. Every mark-scheme answer uses the phrase “mobile ions” or “ions free to move.”

The ionic lattice in the solid state

Solid ionic compounds such as sodium chloride consist of a giant lattice of alternating positive and negative ions. Each Na+ ion is surrounded by Cl- ions and vice versa, held together by strong electrostatic attraction between oppositely charged ions. This is the ionic bond.

In the solid, ions occupy fixed positions. They vibrate but cannot move from place to place. Since electrical conduction requires the movement of charged particles, and no charged particles can move, the solid does not conduct.

State	Ion arrangement	Can ions move?	Conducts?
Solid	Fixed lattice positions	No	No
Molten (liquid)	Lattice broken, ions free	Yes	Yes
Aqueous solution	Ions separated by water	Yes	Yes

What happens on melting

When an ionic compound is heated to its melting point, the thermal energy overcomes the electrostatic forces holding ions in their fixed positions. The lattice breaks down and the ions become free to move throughout the liquid.

When a potential difference is applied across the molten compound, positive ions (cations) move toward the negative electrode (cathode) and negative ions (anions) move toward the positive electrode (anode). This movement of charged particles is an electric current.

For example, molten lead(II) bromide, PbBr2:

• At the cathode: Pb2+ + 2e- -> Pb (lead metal deposited)
• At the anode: 2Br- -> Br2 + 2e- (bromine gas released)

This is electrolysis in action, and it only works because the ions are mobile.

What happens on dissolving

When an ionic compound dissolves in water, water molecules surround individual ions and pull them away from the lattice. The ions become hydrated and dispersed throughout the solution.

Dissolved ions are just as free to move as molten ions. When electrodes are placed in the solution and connected to a power supply, cations migrate to the cathode and anions migrate to the anode. The solution conducts.

The electrolysis of aqueous solutions introduces an additional complication: water itself provides H+ and OH- ions, so the products at the electrodes may differ from those obtained with the molten compound.

Why covalent compounds do not conduct

Covalent compounds such as sugar (C12H22O11) dissolve in water but do not conduct. The reason is that they dissolve as molecules, not ions. No ions are produced, so there are no mobile charge carriers.

This comparison is a common exam question:

Substance	Particles when dissolved	Conducts in solution?
NaCl (ionic)	Na+ and Cl- ions	Yes
C12H22O11 (covalent)	C12H22O11 molecules	No
HCl (covalent but ionises)	H+ and Cl- ions	Yes

Note that HCl is covalent but ionises completely in water, producing ions. This makes hydrochloric acid a conductor despite HCl being a covalent molecule. The distinction between the substance and what it produces in solution is a Supplement-level marking point.

The role of electrons vs ions

Metals conduct electricity by a different mechanism: delocalised electrons move through the metal structure. In ionic conduction, the charge carriers are ions, not electrons. This distinction matters in exam answers.

At the electrodes during electrolysis, there is a transfer between ionic conduction and electronic conduction. Electrons flow through the external circuit (wires), while ions carry the charge through the electrolyte. At each electrode, electrons are either gained (reduction at cathode) or lost (oxidation at anode).

Graphite: a special case

Graphite is a covalent substance that conducts electricity. Each carbon atom bonds to three others, leaving one electron per atom delocalised across the layers. These delocalised electrons, not ions, carry the charge. Graphite conducts like a metal, not like an ionic compound.

This is why graphite is used as electrodes in electrolysis, as explored in giant covalent structures.

Mark-scheme language

The exact wording matters for full marks. Examiners want:

• Solid does not conduct: “ions are in fixed positions / cannot move / no mobile ions”
• Molten/dissolved conducts: “ions are free to move / mobile ions carry the charge”
• Do not say: “electrons move” (unless discussing metals or graphite), “atoms move” (ions, not atoms), “ions are released” (they become free to move, they are not created)

A common 1-mark error is writing “the ions can move” without specifying that they carry charge. The full answer links movement to charge transfer.

Exam-style worked example

Sodium chloride does not conduct electricity as a solid but does conduct when molten. Explain why. [3 marks]

• In solid sodium chloride, the ions are held in fixed positions in the lattice [1]
• They cannot move to carry charge [1]
• When molten, the lattice breaks down and the ions (Na+ and Cl-) are free to move and carry charge / act as mobile charge carriers [1]

Examiner note: Writing “the ions can flow” or “ions are released” typically earns partial credit. The cleanest phrasing is “ions are free to move.”

Explain why a solution of sodium chloride conducts electricity but a solution of glucose does not. [3 marks]

• Sodium chloride is ionic; when dissolved, it produces Na+ and Cl- ions that are free to move [1]
• These mobile ions carry charge through the solution [1]
• Glucose is covalent; it dissolves as molecules, not ions, so there are no mobile charge carriers [1]

Summary checklist

1. Conduction requires mobile charge carriers.
2. In ionic compounds, the charge carriers are ions (not electrons).
3. Solid: ions fixed in lattice, no conduction.
4. Molten: lattice broken, ions free to move, conduction occurs.
5. Dissolved: ions separated by water molecules, free to move, conduction occurs.
6. Covalent molecular substances dissolve as molecules, not ions, so they do not conduct.