The electrical conductivity of ionic substances
3 marks · NormalA student is asked to test whether lithium bromide conducts electricity in three different states:
- solid lithium bromide
- molten (melted) lithium bromide
- lithium bromide dissolved in water
Explain, using the idea of moving ions, in which of these states lithium bromide will conduct electricity. Show your reasoning for each case.
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
Lithium bromide only conducts in molten and aqueous states because the ions are free to move. In the solid state, ions are fixed and cannot move.
Must show: solid: ions fixed, does not conduct, molten: ions free, conducts, aqueous: ions free, conducts
Also accept: Lithium bromide conducts electricity when molten or in aqueous solution because the ions are free to move and carry charge. In solid state, ions are fixed in the lattice and cannot move, so it does not conduct.
Common mistakes: Mentioning electrons instead of ions; Saying all three states conduct
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Identifies both molten and aqueous states will conduct, but solid does not.
M2 · 1 mark · Explains ions are fixed in the solid so cannot carry charge.
M3 · 1 mark · Explains ions are free to move in molten/aqueous states, allowing conduction.
Worked solution
Solid lithium bromide does not conduct electricity because its ions are fixed in the lattice and cannot move (M1, M2).
Molten and aqueous lithium bromide do conduct electricity because in these states, the ions are free to move and can carry charge (M1, M3).