Final answer:
Ionic compounds are insulators when solid due to a rigid lattice structure, but they conduct electricity when molten because this lattice breaks down, allowing ions to move freely.
Step-by-step explanation:
When a substance does not conduct electricity in solid form but does in molten form, it is typically an ionic compound. In solid form, such substances are hard, brittle, and have high to very high melting points. The ions in a solid ionic compound are locked in a rigid lattice structure, preventing them from moving freely. This rigidity means that there are no charge carriers available to support an electric current, making the solid an insulator.
In contrast, when an ionic compound is heated to its melting point, it becomes molten. In this liquid state, the lattice breaks down, and the ions are no longer fixed in place. They can move around and are free to conduct electricity because moving charged particles — in this case, ions — constitute an electric current. Thus, molten ionic compounds, as well as solutions of ionic compounds in water, can conduct electricity effectively.
Examples of such behavior include table salt (sodium chloride) and many other salts, which are insulators when solid but become good conductors as liquids. This distinction is important in understanding the nature of ionic bonds and the conditions required for conductivity.