Question

Ionic solids have high melting point and boiling point. Explain.

Answer 1

Hint: Melting and boiling point depends upon force of attraction between constituent particles. Higher the force of attraction, higher is the melting and boiling point. Ionic solids have a large array of positive and negative ions held together by electrostatic force of attraction.

Complete answer:
Ionic solids are formed by three dimensional arrangements of positive charged ions i.e. cations and negative charged ions i.e. anions. These ions are held together by coulombic(electrostatic) forces and hence, ionic solids are hard and brittle.
Energy must be transferred to a substance to make it melt or boil. This energy overcomes the strong electrostatic forces of attraction which act in all directions between the oppositely charged ions.
The more energy needed, the higher the melting point or boiling point. As the ionic lattice contains such many ions and the electrostatic forces of attraction between oppositely charged ions are strong, 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. Hence, the melting point of MgO is greater than NaCl.

Note:
In solid state, ions of ionic solid are not free to move. Hence, they do not conduct electricity in solid state. However, in the molten state or aqueous solution the ions become free to move and hence, ionic solids conduct electricity in molten or aqueous solution.