Ionic substances dissociate completely into constituent ions when dissolved in water. The energy of attraction between an ion and a water molecule is due to an ion-dipole force. Water molecules are dipolar in nature, so they tend to orient with respect to nearby ions. In the case of a positive ion, water molecules orient with their oxygen atoms (negative pole) towards the ion. In the case of a negative ion, water molecules orient with their hydrogen atoms (positive pole) towards the ion. The process of attraction of ions with water molecules is called hydration. Hydration of ions favour the dissolution of ionic solids in water.
NaCl (s) → Na+ (aq.) + Cl- (aq.)