According to Arrhenius concept,
An acid is a substance which can furnish hydrogen ions in its aqueous solution.
A base is a substance which can furnish hydroxyl ions in its aqueous solution.
STRONG AND WEAK ACIDS AND BASES
Acids such as HCl, H2SO4 and HNO3, which are almost completely ionized in aqueous solutions are termed as strong acids whereas acids such as ethanoic acid (CH3COOH) and trioxocarbonate(IV) acid CH2CO3) which are weakly ionized are called weak acids.
Similarly, bases which are almost completely ionized in aqueous solution are called strong bases, for example, NaOH and KOH. The bases such as NH4OH are only slightly ionized and are called weak bases.
You will learn more about strength of acids and bases in Unit 24.
According to Arrhenius theory, neutralization of acids and bases is basically a reaction between H+ and OH- ions in solutions.
Nature of Hydrogen Ion and Hydroxyl Ion in Aqueous Solution
Hydrogen atom contains one proton and one electron. H+ ion is formed by loss of this electron. Therefore, H+ ion is simply a proton. Charge density of this unshielded proton is very high. Therefore, it is not likely to exist independently as H+ ion. In an · aqueous solution H+ ion is considered to be present in hydrated form in combination with a water molecule as H3O+. H+ + H2O àH3O+
In aqueous solution hydronium ion is further hydrated to give species such as H5O2 +, H7O3 + and H9O4 + Similarly, hydroxyl ion is hydrated to give species such as Hp2-, H5O3, etc.