The ionic compounds contain ions (cations and anions) which are held together by the strong electrostatic forces of attraction.
Ionic compounds are crystalline solids. In the crystal, the ions are arranged in a regular fashion. The ionic compounds are hard and brittle in nature.
Ionic compounds have high melting and boiling points. The melting point of sodium chloride is 1074 K (801°C) and its boiling point is 1686 K (1413°C). The melting and boiling points of ionic compounds are high because of the strong electrostatic forces of attraction present between the ions. Thus, it requires a lot of thermal energy to overcome these forces of attraction.
The thermal energy given to the ionic compounds is used to overcome the inter-ionic attractions present between the cations and anions in an ionic crystal. The crystal has a three dimensional regular arrangement of cations and anions which is called crystal lattice. On heating, the breaking of this crystal lattice leads to the molten state of the ionic compound in which the cations and anions are free to move.
Ionic compounds conduct electricity in their molten state and in aqueous solutions. Since ions are free to move in the molten state, they can carry current from one electrode to another in a cell. Thus ions can conduct electricity in molten state.
However, in solid state, such a movement of ions is not possible as they occupy fixed positions in the crystal lattice. Hence in solid state, ionic compounds do not conduct electricity.
In aqueous solution, water is used as a solvent to dissolve ionic compounds. It weakens the electrostatic forces of attraction present among the ions. When these forces are weakened, the ions become free to move, hence they can conduct electricity.
Ionic compounds are generally soluble in water but are insoluble in organic solvents such as ether, alcohol, carbon tetrachloride, etc. However, a few ionic compounds are insoluble in water due to strong electrostatic force between cation and anion. For example barium sulphate, silver chloride and calcium fluoride.