Arrhenius theory of acids and bases: 

Swante Arrhenius, in 1887, put forward the hypothesis of ions to account for the fact that the products of electrolysis appear only at the electrodes. His theory is called the theory of electrolytic dissociation. According to this theory, a molecule of an electrovalent compound (acid, base or salt) when dissolved in a proper solvent ( usually water ) splits into two parts. One is positively charged, known as ‘Cation’ or positive ion and the other is negatively charged, known as anion or negative ion. Arrhenius defined acids, alkalies and bases as follows :

      a.  Acid –A hydrogen containing compound, which liberates H+ ions in water is called an acid. For example, HCl, H2SO4,HNO3 .

      b.  Alkali –A hydroxyl compound which liberates OH− ions in water is called an alkali. For example, NaOH, KOH.

      Note :    These definitions are applicable to water soluble compounds only.

      c.  Base –A substance which reacts with an acid to form salt is called a base. For example, CaO, MgO. A base which is soluble in water is called an alkali. For example, NaOH, KOH

 

(a)          Neutralisation reaction –The reaction of an acid with an alkali (or base) to form salt and water is called neutralization. For example, HCl(Aq ) + NaOH(Aq ) → NaCl(Aq ) + H2O.

(b)          Strong acid –An acid which dissociates in water to a large extent (about more than 60%) is called a strong acid. For example, HCl, H2SO4,HNO3.

(c)           Weak acid –An acid which dissociates in water to a small extent (about less than 30%) is called a weak acid. For example, HCN, CH3COOH.

(d)          Strong base –A base which dissociates in water to a large extent ( about more than 60%) is called a strong base . For example, NaOH, KOH

(e)          Weak base –A base which dissociates in water to a small extent (about less than 30%) is called a weak base . For example, NH4OH,Ca(OH)2

When a strong acid is just completely reacted with a strong base, the resulting solution is neutral. This solution has no effect on the blue as well as red litmus paper. However, if the acid is strong and base is weak or vice versa then the resulting solution is weakly acidic or weakly alkaline.