Question

Classify each of the following solids as metallic, network covalent, ionic, or molecular. (a) It is insoluble in water, melts above \(500^{\circ} \mathrm{C}\), and does not conduct electricity either as a solid, dissolved in water, or molten. (b) It dissolves in water but does not conduct electricity as an aqueous solution, as a solid, or when molten. (c) It dissolves in water, melts above \(100^{\circ} \mathrm{C}\), and conducts electricity when present in an aqueous solution.

Short answer

Question: Based on their properties, classify each of the following solids as metallic, network covalent, ionic, or molecular: (a) Insoluble in water, melts above \(500^{\circ} \mathrm{C}\), does not conduct electricity as a solid, dissolved in water, or molten. (b) Dissolves in water but does not conduct electricity as an aqueous solution, as a solid, or when molten. (c) Dissolves in water, melts above \(100^{\circ} \mathrm{C}\), and conducts electricity when in an aqueous solution. Answer: Solid (a) is a network covalent solid, solid (b) is a molecular solid, and solid (c) is an ionic solid.

Step-by-step solution

(Identify properties of each solid)

Given the properties of each solid, form a basic understanding of the general characteristics of metallic, network covalent, ionic, and molecular solids.

(Classify Solid (a))

For solid (a), it is insoluble in water, melts above \(500^{\circ} \mathrm{C}\), and does not conduct electricity either as a solid, dissolved in water, or molten. This solid exhibits characteristics typical of network covalent solids, as they are generally high-melting-point compounds and do not conduct electricity. Therefore, solid (a) is a network covalent solid.

(Classify Solid (b))

For solid (b), it dissolves in water but does not conduct electricity as an aqueous solution, as a solid, or when molten. This indicates that the solid is most likely a molecular solid, as these solids often dissolve in water but do not conduct electricity in any state. Therefore, solid (b) is a molecular solid.

(Classify Solid (c))

For solid (c), it dissolves in water, melts above \(100^{\circ} \mathrm{C}\), and conducts electricity when present in an aqueous solution. This is a common characteristic of an ionic solid, as they generally dissolve in water and can conduct electricity when in aqueous solutions because the ions are free to move. Therefore, solid (c) is an ionic solid. In conclusion, solid (a) is a network covalent solid, solid (b) is a molecular solid, and solid (c) is an ionic solid.

Key concepts

Metallic Solids

Metallic solids are materials made entirely of metal atoms. These solids are excellent conductors of electricity and heat due to the presence of free electrons, often described as being "delocalized." This sea of electrons allows metallic solids to conduct electricity easily, even when solid. Because of this property, metallic solids are utilized in many applications, such as wiring and electronics.

These solids typically have a shiny and lustrous appearance, which is due to the electron interactions at the surface. Additionally, metallic solids can be quite malleable and ductile, meaning they can be shaped or drawn into wires without breaking.

Some common examples of metallic solids include metals like copper, iron, and aluminum. These metals typically melt at a wide range of temperatures depending on the type but are iconic for their electrical conductivity and distinctive properties of malleability.

Network Covalent Solids

Network covalent solids are made up of a continuous, three-dimensional network of covalently bonded atoms. These solids are known for their incredibly high melting points, often above 1000°C, as it requires a significant amount of energy to break the covalent bonds present throughout the structure.

Due to the strength and directionality of these bonds, network covalent solids tend to be very hard and durable but typically lack electrical conductivity. This is because there are no free-moving electrons or ions to carry charge. Instead, all electrons are tightly bound in localized covalent bonds.

Well-known examples include diamond and quartz. Diamond, for instance, is composed entirely of carbon atoms each covalently bonded to four other carbon atoms forming a strong lattice, contributing to its legendary hardness.

Ionic Solids

Ionic solids are composed of a continuous lattice of ions held together by strong ionic bonds. These materials as solids are electrical insulators but become good conductors when molten or dissolved in water. This conductivity is due to the mobility of ions in these states.

One of the hallmark properties of ionic solids is their high melting point. This is because substantial energy is needed to overcome the strong electrostatic forces between oppositely charged ions in the solid.

Typically, ionic solids are also brittle. When stress is applied, like charges can be forced next to each other, causing repulsion and cracking across planes of ions. Common examples include sodium chloride (table salt) and magnesium oxide, which are essential in a wide variety of chemical applications.

Molecular Solids

Molecular solids consist of molecules held together by relatively weak forces, such as van der Waals forces, dipole-dipole interactions, or hydrogen bonds, rather than covalent or ionic bonds. Because the forces binding them together are weak, molecular solids generally have low melting points.

These solids are often soft and may be soluble in some solvents but do not conduct electricity, as there are no free charges to carry current. Their low conductivity persists whether solid, in solution, or molten.

Molecular solids can include substances like ice, dry ice (solid CO₂), and various organic compounds such as sugar. These solids are crucial in biological systems and various chemical processes, providing the structural scaffolding or substrates for reactions.