Properties of ionic compounds

The physical properties of ionic compounds can be explained in terms of their structure and bonding.

High melting points and boiling points

curriculum-key-fact
Ionic compounds are solids at room temperature. Melting and boiling are changes of state.

Energy has to be transferred to a substance in order to melt or boil it. This energy is needed to overcome the forces between particles in the substance:

  • some forces are overcome during melting
  • all remaining forces are overcome during boiling

The more energy needed, the higher the melting point or boiling point.

Explanation

Ionic compounds are held together by electrostatic forces between the oppositely charged ions. These forces are usually referred to as ionic bonding. As the ionic lattice contains such a large number of ions, a lot of energy is needed to overcome this ionic bonding so ionic compounds have high melting and boiling points.

The strength of the ionic bonds depends on the charge on the ions. Ions with higher charge will have stronger forces between them, so will need more energy in order to overcome these forces.

CompoundMelting pointBoiling point
NaCl801°C1,413°C
MgO2,852°C3,600°C

Ionic bonding between Mg2+ and O2- ions is stronger than that between Na+ and Cl- ions.

Conduction of electricity

A substance can conduct electricity if:

  • it contains charged particles, and
  • these particles are free to move from place to place

Ionic compounds conduct electricity when melted to form a liquid or are dissolved in water to form an aqueous solution. This is because both processes make their ions free to move from place to place. Ionic compounds cannot conduct electricity when solid, as their ions are held in fixed positions and cannot move.

curriculum-key-fact
Ionic compounds are conductors of electricity when molten or in solution and insulators when solid.
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