A crystalline solid of unknown origin forms an aqueous solution that conducts an electrical current. The solid has a high melting point and shatters when struck with a hammer. The solid is likely to be: (a) A metallic solid (b) A molecular solid (c) An ionic solid (d) A covalent network solid

Understanding the electrical conductivity of solids is crucial for identifying materials and their applications. In our problem, the solid exhibits electrical conductivity when dissolved in water, which is a behavior commonly associated with ionic substances.

Electrical conductivity in solids can arise from the movement of electrons in metallic bonds, or from the flow of ions in an electrolyte solution. Metals have delocalized electrons that freely move throughout the material, allowing them to conduct electricity in the solid state. On the other hand, ionic solids are composed of charged ions held together by electrostatic forces in a crystal lattice. They do not conduct electricity when solid because the ions are fixed in place. However, when dissolved or melted, the ions are free to move, allowing electrical conduction. Molecular and covalent network solids typically do not conduct electricity due to the lack of free charges or delocalized electrons.

To make this clearer: ionic solids only allow charge to flow when the ions can move, which occurs when they're in a liquid state or dissolved, hence our mention of an 'aqueous solution' providing the necessary environment for conductivity.

The physical properties of a solid, such as melting point or brittleness, are determined by the type and strength of the bonds that hold the solid together. Our exercise points out a high melting point and brittleness, which guide us towards classifying the solid.

Ionic solids, characterized by the strong electrostatic forces between ions, generally have high melting points; it takes a significant amount of energy to break these bonds. The solid shattering, rather than deforming, indicates brittleness—a typical property of ionic crystals. This brittleness emerges because when force is applied, like a hammer strike, the aligned charges in the lattice repel each other, causing the solid to break along its planes.

A metallic solid would be malleable and not shatter due to the mobility of the sea of delocalized electrons, which can disperse the applied force. Molecular solids, with their weaker intermolecular forces, typically have lower melting points and can be quite soft. Covalent network solids, such as diamonds, have extremely high melting points and are very hard, but they also do not shatter easily. Understanding these properties helps us determine why ionic solids are distinct from other types of solids.

Crystalline solids can be categorized into four main types: metallic, molecular, ionic, and covalent network solids. Each has a characteristic structure and set of properties.

Metallic solids are made up of atoms surrounded by a 'sea' of delocalized electrons. This structure gives them their ductility, luster, high electrical and thermal conductivity, and the ability to absorb a wide range of energies resulting in opaque but shiny appearances.

Molecular solids consist of molecules held together by intermolecular forces like London dispersion forces, dipole-dipole interactions, or hydrogen bonds. These substances usually have lower melting and boiling points, and they can be transparent or translucent.

Ionic solids, such as table salt, are made up of positively and negatively charged ions in a rigid lattice. They are known for their high melting and boiling points, brittleness, and conduction of electricity in the molten state or when dissolved in water.

Finally, covalent network solids are formed by a network of covalent bonds, creating a robust three-dimensional structure. Examples include diamonds and quartz, which are very hard, have extremely high melting points, and are often transparent.