Giant ionic structures
4 marks · NormalThe table below gives the melting points and ability to conduct electricity in the solid and molten states for some ionic compounds:
| Compound | Melting point (°C) | Conducts electricity when solid? | Conducts electricity when molten? |
|---|---|---|---|
| Sodium chloride | 801 | No | Yes |
| Magnesium oxide | 2852 | No | Yes |
| Potassium bromide | 734 | No | Yes |
Which compound has the highest melting point?
Suggest a reason why all three compounds do not conduct when solid.
What allows these compounds to conduct when molten?
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) Magnesium oxide · must show Name of magnesium oxide
(b) Because their ions are fixed in place in the lattice and cannot move to carry charge. · must show Ions fixed, Cannot move
(c) The ions are free to move. · must show Ions can move
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies magnesium oxide as the compound with the highest melting point.
M1 (b) · 1 mark · States or explains that the ions are fixed in place (in solid).
A1 (b) · 1 mark · Says ions cannot move, so cannot conduct electricity.
B2 (c) · 1 mark · States ions can move in molten state, so compound conducts.
Worked solution
a) Magnesium oxide has the highest melting point (2852°C).
b) All three compounds do not conduct when solid because the ions are locked in the lattice and cannot move, so no current flows.
c) When molten, the ions are free to move and carry an electric charge, so the compounds conduct electricity.