Both covalent-network solids and ionic solids can have melting points well in excess of room temperature, and both can be poor conductors of electricity in their pure form. However, in other ways their properties are quite different. $$ \begin{array}{l}{\text { (a) Which type of solid is more likely to dissolve in water? }} \\ {\text { (b) Which type of solid can become a considerably better }} \\ {\text { conductor of electricity via chemical substitution? }}\end{array} $$

Lets first understand the main differences between covalent-network solids and ionic solids, in terms of their bonding and properties. Covalent-network solids consist of atoms bonded by covalent bonds, forming a three-dimensional network. Some examples include diamond and silicon dioxide. These solids typically have high melting and boiling points due to the strong covalent bonds and poor electrical conductivity as there are no free mobile charge carriers. Ionic solids, on the other hand, are composed of oppositely charged ions which are held together by electrostatic (ionic) forces. Some examples are sodium chloride (table salt) and magnesium oxide. These solids also have high melting and boiling points due to the strong electrostatic forces between the ions, and are poor electrical conductors in their pure form as the ions are not mobile.

The process of dissolving involves breaking the bonds within the solid and forming new bonds or interactions with the solvent (in this case, water). Water is a polar solvent and dissolves ionic compounds through the process of solvation, where the positive and negative ends of the water molecules (hydrogen and oxygen, respectively) interact with the oppositely charged ions in the ionic solid, breaking the ionic bonds and surrounding the individual ions. On the other hand, covalent-network solids possess strong covalent bonds that are not easily broken by water molecules. Moreover, since the covalent-network solid has a nonpolar nature, it does not interact well with the polar solvent (water). Hence, the answer to (a) is: Ionic solids are more likely to dissolve in water due to their ionic nature and interactions with the polar solvent.

Chemical substitution involves replacing a certain atom or ion in the solid structure with another, which can change the electronic properties of the solid. In this context, we are looking for which type of solid can become a better conductor of electricity via chemical substitution. In ionic solids, chemical substitution usually involves replacing an ion with another ion of the same charge. This could change the solid's electrical properties but does not necessarily make it a considerably better conductor of electricity, as the ions remain immobile unless the solid is melted or dissolved in a solution. For covalent-network solids, chemical substitution can involve replacing an atom with another that has a different number of valence electrons (doping, as in the case of semiconductors). This introduces extra electrons or "holes" (the absence of an electron) into the solid which can move through the structure and contribute to electrical conduction, making the covalent-network solid a considerably better conductor. Hence, the answer to (b) is: Covalent-network solids can become a considerably better conductor of electricity via chemical substitution (e.g., doping in semiconductor materials).