Problem 65 - Chemistry: The Central Science

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. (a) Which type of solid is more likely to dissolve in water? (b) Which type of solid can become an electrical conductor via chemical substitution?

Step-by-Step Solution

Answer

(a) Ionic solids are more likely to dissolve in water because water molecules can interact with and surround the ions, breaking the ionic bonds and causing the solid to dissolve. (b) Ionic solids can become electrical conductors via chemical substitution through a process called doping, in which a small amount of an impurity with a different charge is added to the ionic solid, creating mobile charge carriers within the solid.

Step 1: a) Solubility in water

Covalent-network solids are made of a three-dimensional network of covalently bonded atoms. These types of solids are generally insoluble in water due to the strong covalent bonds between atoms in the solid, making it difficult for water molecules to break apart and interact with the atoms in the solid. Examples of covalent-network solids include diamond, graphite, and quartz. Ionic solids, on the other hand, are composed of positively and negatively charged ions held together by ionic bonding. In general, ionic solids are more likely to dissolve in water because water molecules can interact with and surround the ions, breaking the ionic bonds and causing the solid to dissolve. Therefore, ionic solids are more likely to dissolve in water.

Step 2: b) Electrical conductivity via chemical substitution

Covalent-network solids are poor conductors of electricity in their pure form due to the absence of free-moving charged particles. It is difficult to increase their conductivity via chemical substitution, as this would require the introduction of mobile charge carriers within the rigid covalent network. Ionic solids, however, can become electrical conductors via chemical substitution. This can be done through a process known as doping, in which a small amount of an impurity with a different charge is added to the ionic solid. This leads to the formation of vacancies or interstitials, creating mobile charge carriers within the solid. For example, when an impurity is introduced into a pure ionic solid, the extra electrons or holes can move through the solid, increasing its electrical conductivity. Therefore, ionic solids can become electrical conductors via chemical substitution.