Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.

Let's tackle question 15. a).

15. a) At 25°C, sodium chloride is a solid. In the solid state, its ions ($\text{Na}^+$ and $\text{Cl}^-$) are held in fixed positions within the crystal lattice and are not free to move, so it cannot conduct electricity. Between 801°C and 1413°C, sodium chloride is molten. In the molten state, the ions are mobile and free to move, allowing it to conduct electricity.