Why things conduct electricity
5 marks · HardA teacher tests the electrical conductivity of four substances in different states. The results are shown in the table below.
| Substance | State | Conducts electricity? |
|---|---|---|
| Potassium nitrate | Solid | No |
| Potassium nitrate | Aqueous | Yes |
| Sulfur dioxide | Liquid | No |
| Lead(II) bromide | Molten | Yes |
Identify which of the substances in the table are ionic compounds.
Explain why potassium nitrate conducts electricity when dissolved in water but not as a solid.
Suggest why sulfur dioxide does not conduct electricity, even when liquid. Use your knowledge of bonding.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Potassium nitrate, Lead(II) bromide
(b) Potassium nitrate conducts when dissolved because its ions are free to move in solution. In solid form, ions are held in a lattice and cannot move, so it does not conduct.
(c) Sulfur dioxide is a covalent compound. It does not have ions to carry charge, so even when liquid, it does not conduct electricity.
Common mistakes: Assuming all molten or liquid substances conduct; Incorrectly labelling bonding types
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies both potassium nitrate and lead(II) bromide as ionic compounds.
M1 (b) · 1 mark · States that potassium nitrate conducts in solution because ions are free to move.
M2 (b) · 1 mark · Explains that ions are fixed in a solid lattice, so solid does not conduct.
M3 (c) · 1 mark · Recognises sulfur dioxide is covalent and does not have ions.
M4 (c) · 1 mark · Links the lack of ions to no conduction even when liquid.
Worked solution
a) Potassium nitrate and lead(II) bromide are ionic compounds.
b) In solid potassium nitrate, the ions are fixed in the lattice and cannot move, so it does not conduct. When dissolved in water, the ions are free to move, so it can carry electric current.
c) Sulfur dioxide is a covalent compound, so it does not have ions. Even in the liquid state, there are no charged particles to carry current, so it does not conduct.