Question

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. $$\begin{array}{l}\text{ (a) Which type of solid is more likely to dissolve inwater? }\\ \text{ (b) Which type of solid can become a considerablybetter }\\ \text{ conductor of electricity via chemical substitution? }\end{array}$$

Short answer

(a) Ionic solids are more likely to dissolve in water due to their ionic nature and interactions with the polar solvent. (b) Covalent-network solids can become a considerably better conductor of electricity via chemical substitution (e.g., doping in semiconductor materials).

Step-by-step solution

Understand covalent-network and ionic solids

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.

Determine which type of solid dissolves in water

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.

Determine which type of solid can conduct electricity via chemical substitution

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).

Key concepts

Covalent-Network Solids

Covalent-network solids are substances where atoms are connected in a continuous network by covalent bonds. These solids are known for their extreme hardness and high melting points. This is due to the robust covalent bonds that span throughout the entire structure, creating a three-dimensional lattice. Examples of covalent-network solids include diamond, where each carbon atom is bonded to four other carbon atoms, and silicon dioxide (quartz), which features silicon and oxygen atoms engaged in strong covalent bonding.
One characteristic of covalent-network solids is that they are generally poor conductors of electricity. This is because there are no free electrons or ions to transport charge. In these structures, all valence electrons are tied up in strong covalent bonds, which means there are no mobile charge carriers available to facilitate conductivity. This lack of electrical conduction is a distinguishing feature compared to other types of solids.

• Covalent bonds create a rigid structure.
• Usually poor electrical conductors.
• Strong and durable with high melting points.

Ionic Solids

Ionic solids are composed of positively and negatively charged ions. These ions are held together by strong electrostatic forces, often referred to as ionic bonds. Sodium chloride (NaCl), common table salt, is a classic example of an ionic solid. The bonding in ionic solids results in a crystalline structure that is hard, yet brittle. The brittleness arises because when the lattice structure is disrupted, similar charges align and repel each other.
Although ionic solids tend to have high melting and boiling points due to the strength of the ionic bonds, their electrical conductivity in solid form is quite low. This is because the ions are fixed in place within the crystal lattice and cannot move to conduct electricity. However, when dissolved in water or melted, ionic compounds can conduct electricity efficiently. This is because the ions become free to move and carry charge.

• Composed of charged ions held by electrostatic forces.
• High melting and boiling points.
• Conducts electricity when dissolved or melted.

Electrical Conductivity

Electrical conductivity is the measure of a material's ability to conduct an electric current. In the context of solids, it depends on the presence and mobility of charge carriers such as electrons or ions. Both covalent-network and ionic solids generally have low electrical conductivity in their pure forms.
In ionic solids, the lack of mobility for ions in a solid state restricts electrical conductivity. Only when these solids are either melted or dissolved do they conduct electricity effectively, thanks to the free movement of ions.
In contrast, covalent-network solids can often become more conductive through a process called doping. Doping involves adding impurities to the solid that introduce extra charge carriers into the structure. For instance, adding phosphorus or boron to silicon modifies its electrical properties by introducing free electrons or creating "holes", enhancing its ability to conduct electricity.

• Depends on the availability and mobility of charge carriers.
• Ionic solids conduct when ions are free to move in a solution or liquid state.
• Covalent solids can be modified (doped) to improve conductivity.