Can someone explain why we can smell a dilute solution of ammonia but not a dil solution of sodium hydroxide?

Answer 1

Sodium hydroxide remains ionic and in solution. Ammonia in water reacts to form Ammonium hydroxide, but this is in equilibrium with NH3 and H2O:

NH3 + H2O = (NH4)+ + (OH)-

It is the Ammonia gas escaping that give a dilute Ammonium hydroxide solution its smell.