Class 10 Science


Dissociation of acid in aqueous solution

These notes are based on the chapter Acids Bases and Salts from NCERT book based on CBSE syllabus.

You have read that acids give hydrogen gas when they react with metal. This shows that all acids contain hydrogen. For example: Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), etc.

Water is necessary for acid to show acidic behaviour

When an acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in aqueous solution is the common property in all acids.

The acidic behaviour of acid is because of its ability to dissociate hydrogen ion in aqueous solution. An acid does not dissociate hydrogen ion in absence of water.

When you bring blue litmus paper in contact with dry hydrochloric acid, there is no change in colour of litmus paper. Why?

That is why, if you will bring blue litmus in contact with dry hydrochloric acid there will be no change in colour of litmus paper. But if a wet blue litmus paper is brought in contact with dry hydrochloric acid then blue litmus paper turns red due to dissociation of hydrogen ion in presence of moisture. This simple experiment shows the necessity of water for an acid to show acidic behaviour.

Dissociation of HCl in water

Example 1: Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl) when it is dissolved in water.

HCl (aq) ⇨ H+ + Cl

Dissociation of H2SO4 in water

Example 2: Sulphuric acid (H2SO4) gives hydrogen ion (H+) and sulphate ion (SO4− −) in water.

H2SO4 (aq) ⇨ H+ + SO4− −

Dissociation of HNO3 in water

Example 3: Nitric acid (HNO3) gives hydrogen ion (H+) and nitrate ion (NO3) in water.

HNO3 (aq) ⇨ H+ + NO3

Dissociation of CH3COOH in water

Example 4: Acetic acid (CH3COOH) gives acetate ion (CH3COO) and hydrogen ion (H+).

CH3COOH (aq) ⇨ CH3COO H+

Hydrogen ion which is produced by acid (when acid is combined with water molecule), exists in the form of hydronium ion (H3O) in aqueous solution. That’s why hydrogen ion is always written with suffix (aq), such as H+ (aq).

HCl + H2O ⇨ H3O + Cl

H2SO4 + H2O ⇨ H3O + SO4− −

Thus, because of dissociation of hydrogen ions, acid shows its acidic behavior.

Why does an acid conduct electricity in aqueous solution?

Acids conduct electricity in their aqueous solution because of dissociation of hydrogen ion. Hydrogen ion in aqueous solution conducts electricity.

Acidic behavior of carbon dioxide gas: Carbon dioxide gas produces carbonic acid when dissolved in water. This carbonic acid dissociates hydrogen ion and carbonate ion in the aqueous solution.

CO2 + H2O ⇨ H2CO3 ⇨ H+ + CO3− −

Are all compounds which contain hydrogen, necessarily acids?

No, all compounds which contain hydrogen are not acid. For example, glucose (C6H12O6), methyl alcohol (CH3OH), etc. are not acid in spite of the fact that they contain hydrogen. This is because these compounds do not dissociate hydrogen ion in their aqueous solution.


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