Passing electricity through compounds: electrolysis
3 marks · NormalExplain why molten magnesium chloride ( ) conducts electricity, but solid magnesium chloride does not. Use your knowledge of ionic compounds and their structure.
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
Molten magnesium chloride conducts electricity because its ions are free to move, but in solid magnesium chloride the ions are held in fixed positions and cannot move.
Must show: mention of ions as charge carriers, fixed lattice in solid, free movement in melt
Also accept: Ions are only able to move when molten, not in solid
Common mistakes: Thinking electrons move in the solid; Saying ions are absent in solid state
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Mentions that electrical conduction requires free-moving ions
M1 · 1 mark · Explains that in solid magnesium chloride the ions are held in a fixed lattice (cannot move)
A1 · 1 mark · States that in molten magnesium chloride, ions are free to move around
Worked solution
In solid , magnesium and chloride ions are held in a fixed lattice—they can only vibrate and are not free to move. Because the ions cannot move, the solid does not conduct electricity.
When molten, the ionic lattice breaks down and the ions are free to move. These free ions carry the electric current through the liquid, so molten conducts electricity.