Acids are proton donors. Bases are proton acceptors. Acids and bases react by proton transfer.
H2O can act as both an acid and a base: H2O(l) + H2O(l)
H3O+ + OH–
Substances referred to as acids are stronger than water.
The reaction of strong acids with water is product-favoured (in these cases all of the acid dissolved forms products).
Reaction of weak acids with water is reactant-favoured (the extent depending on the acid).
How do I know if an acid is weak or strong?
These are the Bronsted-Lowry definitions.
H2O can act as both an acid and a base: H2O(l) + H2O(l)
H3O+ + OH–This reaction is described as strongly reactant favoured because it occurs to a very small (but measureable) extent in pure water.
Substances referred to as acids are stronger than water.
Strong acids are stronger than water and stronger than H3O+
Weak acids are stronger than water and weaker than H3O+
Weak acids are stronger than water and weaker than H3O+
| pH = –log[H3O+] | aqueous solution has no effect on litmus | aqueous solution turns blue litmus red and has a lower pH than H2O | ||||
acid | CH3OH not acidic | H2O | NH4+ weak acid |  weak acid | H3O+ | HCl, HBr,HI HNO3 H2SO4 HClO4 strong acids |
The reaction of strong acids with water is product-favoured (in these cases all of the acid dissolved forms products).
HCl(aq) + H2O 
H3O+ + Cl– (aq)
The H3O+ concentration or [H3O+] in the aqueous solution of a strong acid is equal to the concentration of the acid.
Thus the pH can be directly calculated from the concentration.
H3O+ + Cl– (aq)The H3O+ concentration or [H3O+] in the aqueous solution of a strong acid is equal to the concentration of the acid.
Thus the pH can be directly calculated from the concentration.
For 1 HCl, [H3O+] is also 1 and the pH is 0.
Reaction of weak acids with water is reactant-favoured (the extent depending on the acid).
[H3O+] in their aqueous solutions is lower than the initial concentration of the weak acid.
The pH of these solutions is higher than a strong acid in the same concentration.
The pH of these solutions is higher than a strong acid in the same concentration.
How do I know if an acid is weak or strong?
There are very few common strong acids. The formula for these are given in the table above. If you learn that these are strong, it is fair to assume that other acids that you may encounter are weak.