What do bases undergo when dissolved in water: ionisation or dissociation or both? Some books say ionisation while others say dissociation. What is the correct answer?
It really depends on what base you are talking about. A Lewis base, like an amine, does not dissociate. Instead, it donates its electron pair to accept a hydrogen from a water molecule, forming its conjugate acid and a hydroxide ion.
An Arrhenius base, like sodium hydroxide, which is already composed of sodium ions and hydroxide ions will dissociate into free ions in water. To be clear—though in the solid form, the sodium atom has already been ionized, meaning it has donated its outermost electron. It is the electrostatic attraction between positive and negative ions that holds an ionic compound together.