A solid is hard, brittle, and electrically nonconducting. Its melt (the liquid form of the substance) and an aqueous solution containing the substance do conduct electricity. Classify the solid.

Ionic compounds are a class of chemical compounds comprising ions held together by electrostatic forces termed ionic bonding. The structure of an ionic compound is characterized by a regular arrangement of ions known as a crystal lattice. These compounds form when metals react with nonmetals, producing cations (positively charged ions) and anions (negatively charged ions).

For instance, common table salt, or sodium chloride ($NaCl$), is formed from the reaction of sodium (a metal) with chlorine (a nonmetal). Ionic compounds generally exhibit high melting and boiling points due to the strong attractions between oppositely charged ions. They are typically solid at room temperature and are hard and brittle in nature. When dissolved in water or melted, ionic compounds conduct electricity due to the free movement of ions, which act as carriers of electric current.

Electrical conductivity is a measure of how well a material allows the flow of electric current. In the context of solids, conductivity largely depends on the structure and bonding of the material. Solids can be classified based on their ability to conduct electricity: conductors, semiconductors, and insulators.

Metals are excellent conductors due to the presence of free electrons that can move easily within the material. Semiconductor materials have moderate conductivity which can be altered or enhanced by adding impurities, a process known as doping. Insulators, such as covalent network solids (e.g., diamond), do not conduct electricity because they lack free charges for current to flow.

Solids are distinguished by various properties such as hardness, malleability, ductility, electrical and thermal conductivity, and melting point. These properties arise from the types of bonding and the arrangements of atoms within the solid.

Hardness and Brittleness

Hardness refers to a solid’s resistance to deformation or scratching, while brittleness is a measure of how easily a material can fracture when stress is applied. Ionic solids are typically hard and brittle due to the strong ionic bonds holding the lattice together.

Conductivity

The conductive nature of solids can also be indicative of their type. Ionic solids, for example, do not conduct electricity in their solid state because their ions are fixed in place within the lattice and cannot move freely. However, upon melting or dissolving in water, these ions become mobile and can conduct electricity.