APPENDIX B: sIMPlE solutIoN
Acids and bases
the other hand, is a polyprotic acid, 4 because each mol-
ecule has more than one H + to donate.
A base is a substance that accepts or absorbs H + ions,
thereby depleting the host solution in free H + and mak-
ing it less acidic. The reaction between KOH (a base)
and H 2 SO 4 (sulfuric acid) may be written:
The simplest definition of an acid is 'a substance capa-
ble of contributing hydrogen ions to a solution or
reaction' - that is, an H + donor. For example, hydrochlo-
ric acid (HCl) when dissolved in water undergoes
showing that KOH is behaving as a base whose net
effect is to convert H + into H 2 O.
Other reactions between acids and bases show the
In fact this is a simplification. Each ion is actually sur-
rounded in solution by a shell of water molecules
attracted electrostatically by the charge of the ion
( hydration - Box 4.1). One can represent these hydrated
ions as H + aq and Cl − aq .
An acid like HCl, which contributes only one hydro-
gen ion (H + = proton) per acid molecule to the solution,
is said to be monoprotic . Phosphoric acid (H 3 PO 4 ), on
NaOH HCl NaCl HO
+ → +
Sometimes such acids are alternatively described as monobasic
and polybasic respectively.