According to Arrhenius concept An acid is defined as a hydrogen containing substance which gives H+ ion (H3O+) when dissolved in water.

* A base is a substance which contains OH groups and gives hydroxyl ions OH- when dissolved in water.

The concept of Arrhenius is based on the ionic dissociation of the water compound.

FOR Example:

HCL is an acid because it produces H30+ ions in water but CH4 is not.

*NaOH is a base because it furnishes OH- ions whereas C2H5OH is not a base.

*HCl + H2O…………. H3O+   + Cl-
NaOH + H2O………… Na+     + OH-

*According to this theory , all the acid -base reaction are limited to aqueous medium only. It dose not explain the acid base reaction taking place in non -aquous solvents such as liquid ammonia.



An acid is defined as a species (a compound or an ion ) which donates or tends to donate a proton (H+).

A base is defined an a species which accept  or tends to accept a proton.
This dissociation of an acid HA can be represented as:
HA………..H+ + H-

negatively charged ion ( anion) act as base , Thus CH3COO_  is a base  and is said to  be conjugate base  of acetic acid.

* In an acid-base reaction , an acid yields a base (conjugate) and base after accepting proton yields a conjugate acid. This acid -base reaction is represented as:

A1 + B2………..B1 + B2
acid    base       acid     base


HCI + H2O……..CI-    + H3O+
H2O + H2O……..OH-+ H3O+
acid         base              c-base        c-acid

A species that act both as a proton doner and a proton accepter is said to be amphiprotic.

For Example:-

(1) H2O IS Amphiprotic:

It loses proton to a base such as NH3 or accepts a proton from an acid such as HCI.
H2O + H3O………. NH4+  + OH-
acid           base
H2O + H3O…………H2S +H2O
base              acidb


Acids containing one proton which can be donated are monoprotic acids .

Those acid which contain more then one donate able proton are known as Polyprotic Acids.

HCI ,HNO3, HCN etc are the monoprotic acids. whereas H3PO4, H2SO4 are polyprotic acids.


According to Lewis, “Acid is classified as species (molecules or ions) that can accept pair of electrons, and base is species that can give pairs of electrons. 

* An acid is an electrophile (electron loving)
*  Base is a nucleophile (nucleus loving)
*  Acid base reaction involves donation of a pair of electrons from a base to an acid with coordinates bond between the two .

*FOR Example:

Compounds having less than full octect of electrons e.g Bf3 , SO3.



The protonic term can not be extended to species that do not have any prot-ons, such as the oxide network.

*According to the Lux-flood concept.
Base is defined as any species that give up oxide ion (o-2) and
An acid is defined as  any species which gains or takes up oxide ions .
Base………… Acid + oxide (o-2)
For example:-
CaO is a base ,because it gives up o2- ions.
CaO ……….. Ca2+  + O2_


According to this concept an acid is defined as any species,

(1) Which capable of giving cations

(2) Capable with combining with anions

(3) neutralizing a base to give  a salt.

*A base is defined as any species ,

(1) capable of giving up anions or electrons.

(2) combining with cations.

(3) neutralizing an acid to give a salt.

For E,xample:-

ACID             BASE                SALT
CI2                Na                        NaCI

Na lose the electron CI gain the


According to Bronsted , the strength of an acid is measured from it tendency to donate a proton and  that  of a base  from its tendency to accept a proton.

Strengths are generally expressed in terms of expressed of dissociation constants (Ka) and  pka values of acids.

   Let us consider a pro-tonic acid HX. In aqueous solution it constitutes the following equilibrium  HX + H2O —–> X- + H3+O

     According to Law of Mass Action we have:

K=  [X-][H3O+]

Where K is an equilibrium constant concentrations and to be more exact thermodynamic-ally with in the square brackets denote molar concentrations.

As water is always in excess its concentration is constant and
[X-][H3O+]  = K [H2O] = Ka


Ka is called dissociation constant of acid and base represented the extent to which an acid is dissociated.

Greater the value Ka , stronger is the acid and versa .
1/ka is the strength of the X-base.

The strength of an acid can also be expressed in terms of its pka where
pka = log[1   ]   = -log(ka)


A large value of pka means the acid is little dissociated and a small value means that acid is highly dissociated .

Thus HCl is a stronger acid than HNO3  and its dissociation constant (107) is greater than that of HNO3 (103) and its pka value (-7.0) is less than that of HNO3(-3.0).

The strength of an acid is related to that of its conjugate base. The conjugate base is weak if the acid is heavy.

For example ,

HCI is strong acid as it has got a great tendency to lose a proton. Its conjugate base Cl-  ions is a weak base as it has got a little tendency to accept a proton, Reverse is true , if the conjugate base is strong.

The acid strength depends upon the solvent chosen.

 Example ;

In a strongly acidic solvent like H2SO4, the pka value for HCIO4 is very heigh as compared the value in H2O.

All the strong acids like HCIO4, H2SO4, HCI, HNO3 , has very close pka values.