The electrical conductivity of ionic substances
4 marks · NormalExplain, using particles, why solid magnesium chloride does not conduct electricity.
Describe what changes when magnesium chloride melts and how this allows conduction.
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(a) In solid magnesium chloride, magnesium and chloride ions are held in a rigid lattice and cannot move, so there is no conduction of electricity. · must show rigid lattice/structure, ions fixed
(b) When melted, the ionic lattice breaks down and the ions are free to move, allowing them to carry an electric current. · must show lattice broken, ions mobile
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Marking points
M1 (a) · 1 mark · Describes ions held in a lattice or fixed structure.
A1 (a) · 1 mark · States ions cannot move and so electricity is not conducted.
M2 (b) · 1 mark · Describes lattice breaking when melted, ions able to move.
A2 (b) · 1 mark · Ions moving allows conduction of electricity.
Worked solution
(a) The magnesium and chloride ions in solid magnesium chloride are fixed in a rigid lattice and cannot move, so the solid cannot conduct electricity.
(b) When melted, the lattice structure breaks and the ions become free to move about, so magnesium chloride conducts electricity as a liquid.