KEY2CHEM

Properties of Ionic Solids

The regular repetition of ions in an ionic solid has consequences for its properties. Due to the strong interaction between the ions, ionic solids have low vapor pressure and high melting points because it takes significant energy to disrupt the electrostatic attractions holding the particles together. Ionic solids are also hard and brittle (do not bend or deform under external force) due to like-charge repulsions when ions slide past one another.

Example 1.

Which ionic solid is expected to have the highest melting point?

A. $$\require{mhchem}\ce{LiF}$$

B. $$\require{mhchem}\ce{NaCl}$$

C. $$\require{mhchem}\ce{MgO}$$

Solution

C. $$\require{mhchem}\ce{MgO}$$

Ions with higher charges lead to higher electrostatic attractions (arising from larger magnitude charges and smaller ion sizes), and therefore higher melting points.

Example 2.

In which solvent is solid $$\require{mhchem}\ce{NaCl}$$ expected to be the least soluble?

A. $$\require{mhchem}\ce{H2O}$$

B. $$\require{mhchem}\ce{CH3OH}$$

C. $$\require{mhchem}\ce{C5H12}$$

Solution

C. $$\require{mhchem}\ce{C5H12}$$

Ionic compounds tend not to dissolve in nonpolar solvents because the attractions among the ions are much stronger than the attractions among the separated ions and the nonpolar solvent molecules.

Example 3.

Which statement about the conductivity of ionic solids is true?

A. Ionic solids conduct electricity in the solid state.

B. Ionic solids only conduct electricity when dissolved in water.

C. Ionic solids conduct electricity when melted.

Solution

C. Ionic solids conduct electricity when melted.

Ionic solids conduct electricity either when melted or when dissolved in water because the ions are free to move and carry the charge of the current. In the solid state, the ions are fixed in position, and the solid does not conduct electricity.