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Neutral, acidic and basic solutions
We have seen that in pure water H+(aq) and OH–(aq) ions are produced in equal numbers as a result of dissociation of water and therefore, their concentrations are also equal i.e.
[H+] = [OH–]
(i) Neutral solutions
In all neutral aqueous solutions, the concentrations of H+(aq) and OH–(aq) ions remains equal i.e.
[H+] = [OH–]
In other words the neutral solution is the one in which the concentrations of H+ and OH– ions are equal.
(ii) Acidic solutions
Acids furnish H+(aq) ions in their solutions resulting in increase in their concentration. Thus, in acidic solutions
[H+] > [OH–]
And [H+] > 1.0 × 10–7 mol L–1
In other words the acidic solution is the one in which the concentration of H+(aq) is greater than that of OH–(aq) ions.
We have seen earlier that the ionic product of water Kw is constant at a given temperature. It can remain so only if the concentration of OH–(aq) ions decreases. [OH–] < 10–7 mol L–1
(iii) Basic solutions
Bases furnish OH–(aq) ions in their solutions. This results in an increase in their concentration. Therefore, in basic solution
[OH–] > [H+] |
|
and |
[OH–] > 1.0 × 10–7 mol L–1 |
In other words, the basic solution is the one in which the concentration of H+(aq) ions is smaller than that of OH–1(aq) ions.
Here also, because of constancy of ionic product of water Kw, the concentration of H+(aq) decreases. Thus
[H+] < 1.0 × 10–7 mol L–1
We may summarize the nature of aqueous solution in terms of concentration of hydrogen ions H+(aq) as shown in table below.
Concentration of H+(aq) ions in different types of aqueous solutions
Nature of solution |
Concentration of H+ ions |
at 25°C (298 K) |
|
Neutral |
[H+] = 1.0 × 10–7 mol L–1 |
Acidic |
[H+] > 1.0 × 10–7 mol L–1 |
Basic |
[H+] < 1.0 × 10–7 mol L–1 |