Question

You are given a white substance that sublimes at \(3000^{\circ} \mathrm{C}\); the solid is a nonconductor of electricity and is insoluble in water. Which type of solid (Table 11.7) might this substance be?

Short answer

The white substance is most likely a covalent-network solid. This is because it has a high sublimation temperature (3000\(^\circ\)C), does not conduct electricity, and is insoluble in water, which matches the properties of covalent-network solids.

Step-by-step solution

Review the types of solids from Table 11.7

Table 11.7 lists the different types of solids and their properties. They are ionic solids, metallic solids, covalent-network solids, and molecular solids.

Rule out types of solids based on properties given

First, we can rule out ionic solids because they are generally soluble in water and good conductors of electricity. Second, we can rule out metallic solids as they are also good conductors of electricity.

Compare remaining types of solids with given properties

Now we need to analyze the remaining types of solids: molecular solids and covalent-network solids. Molecular solids are composed of molecules or atoms held together by van der Waals forces or hydrogen bonds. They tend to have low melting points, so the high sublimation temperature of 3000\(^\circ\)C does not match with molecular solids. Covalent-network solids consist of atoms bonded together by covalent bonds in a large, extended network. These solids have high melting points, are insoluble in water and do not conduct electricity. Based on these characteristics, the white substance seems to align with the properties of covalent-network solids.

Conclusion

The substance given is most likely a covalent-network solid because it has a high sublimation temperature, does not conduct electricity, and is insoluble in water, which matches the properties of covalent-network solids.

Key concepts

Covalent-Network Solids

Covalent-network solids are a distinctive class of materials characterized by their unique structure and properties. Unlike other types of solids, covalent-network solids consist of a large network of covalent bonds, where atoms are linked together in an extended three-dimensional array. This structure is responsible for their extraordinary hardness and high melting and sublimation points. Sublimation, in this context, refers to the process where a solid transitions directly to a gas phase without passing through a liquid phase, which occurs at very high temperatures for covalent-network solids due to the strong covalent bonds.

Examples of covalent-network solids include diamonds, which are made of carbon atoms bonded together in a tetrahedral lattice, and silicon carbide, which is known for its exceptional thermal stability and hardness. This type of solid is also typically insoluble in water because the strong covalent bonds are difficult to break, and there's no polarity for water molecules to interact with. Students should note that the properties of covalent-network solids are a direct result of their robust and extensive bonding network.

Sublimation Temperature

The sublimation temperature is a critical physical property that refers to the temperature at which a substance transitions from a solid to gas phase without first becoming liquid. This behavior is noted in materials where the vapor pressure exceeds the atmospheric pressure and the temperature is below the melting point. The phenomenon is particularly prevalent in covalent-network solids due to their strong interatomic bonds.

A substance's sublimation temperature can offer insight into its intermolecular forces and stability. For example, a high sublimation temperature, such as 3000°C in the case of some covalent-network solids, suggests very strong bonds between its constituent atoms, typically covalent in nature, which require considerable energy to break. This concept is not only relevant in academic studies but also in practical applications such as freeze-drying, where the ability to sublime is leveraged for preserving biological materials.

Electrical Conductivity in Solids

Electrical conductivity in solids is a property that describes a material's ability to conduct an electric current. This property depends on the presence of mobile charged particles within the material, such as electrons or ions. In metals, for instance, the sea of delocalized electrons allows them to be excellent conductors. However, not all solids conduct electricity well.

Covalent-network solids, in particular, are poor conductors because they lack free-moving charge carriers. The electrons in covalent bonds are localized and not free to move across the structure, preventing the flow of electrical current. Substances like diamond and quartz are examples of such materials. On the other hand, some covalent-network solids, like graphite, can conduct electricity due to the presence of delocalized electrons in one direction. Understanding the electrical properties of solids is essential in materials science and electronics, as it determines the potential use of a material in practical applications, such as semiconductors, insulators, and conductors.