(a) Ionic compounds are aggregates of positively charged ions (cations) and negatively charged ions (anions), held together by very strong electrostatic forces. As the ions are not free to move in any direction, therefore, on the whole an ionic compound is a hard crystalline solid.
(b) Water is a polar covalent compound in which hydrogen atoms are slightly positively charged and oxygen atoms slightly negatively charged. When a number of water molecules align with the cations (positively charged ion) of an ionic compound such that oxygen ions are facing it, then they try to pull the cations out of the ionic bond. Conversely, when the number of hydrogen ions of water surround an anion of an ionic compound, they try to pull the anion out of the ionic bond. Because of these electrostatic pull exerted by water molecules, the ionic bond breaks and hence the ionic compound dissolves in water.
(c) In an aqueous solution or in the fused state, ionic bond between the oppositely charged ions breaks. Thus, the ions are free to move about in all directions.
When electric potential is applied to such a solution, the cations migrate to the cathode and the anions migrate to anode. Due to the migration of ions to the oppositely charged electrodes the solution of ionic compound conducts electricity.