Bases: Bases are metal oxides, hydroxides or compounds (such as NH3) that give OH- ions in aqueous solution.
CuO (s) + 2HCl (aq.) → CuCl2 (aq.) + H2O (l)
NaOH (s) + HCl (aq.) → CuCl2 (aq.) + H2O (l)
H+ + OH- → H2O
NH3 (g) + H2O (l) ↔ NH4+ (aq.) + OH-
Alkali: A base that dissolves in H2O to give OH- ions is called an alkali. NH3, NaOH are examples.
‘All alkali are base, but all bases are not alkali.”