Properties of ionic compounds

The physical properties of ionic compounds can be explained by thinking about their structure and bonding.

High melting points and boiling points

Ionic compounds are solids at room temperature. Melting and boiling are state changes.

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

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

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

Explanation

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

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

Ionic bonds between Mg2+ and O2- ions are stronger than those 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 molten to form a liquid or 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.

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Ionic compounds are conductors of electricity when molten or in solution and insulators when solid.