Question: which two ions are present in pure water?
Answer: The ionic product, Kw
pH.
Variation in Kw with temperature and pressure.
Weak acids. ... 
H2O H+ + OH- ... 
H2O (liq) H+(aq) + OH- (aq)
2 H2O(aq) H3O+(aq) + OH-(aq) ... 
2 H2O(aq) H3O+(aq) + OH-(aq) ΔV = -22.3 cm3 mol-1