International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What would you expect from a crystalline solid composed of ionic compounds?

1. Low melting and boiling points

2. High electrical conductivity when solid

3. High melting and boiling points

4. Unstable at room temperature

The correct answer is: High melting and boiling points

A crystalline solid composed of ionic compounds typically exhibits high melting and boiling points due to the strong electrostatic forces of attraction between the positively charged cations and negatively charged anions. These forces, known as ionic bonds, require a significant amount of energy to overcome. As a result, ionic compounds usually have strong lattice structures, which contribute to their stability at high temperatures. The high melting and boiling points also indicate that a large amount of energy must be supplied to break the ionic bonds when changing from solid to liquid or from liquid to gas. This characteristic is consistently observed in many common ionic compounds, like sodium chloride, which remains solid at room temperature but melts at high temperatures. In contrast, a crystalline structure made of ionic compounds does not exhibit low melting or boiling points, as the forces holding the ions together are quite strong. Likewise, these solids do not conduct electricity in their solid state because the ions are fixed in place within the crystal lattice and cannot move freely. Only when melted or dissolved in water do ionic compounds allow for electrical conductivity due to the mobility of the ions. Additionally, they are generally stable at room temperature, contradicting any notion of being unstable.