Because, acid forms H3O+ ions with water (Separation of H+ ions is possible only in the presence of water).

Example:- HCl + H2O → H3O+ + Cl–

Acid Dissociation in Water

When an acid is added to water, it undergoes a process known as dissociation, where it splits into its constituent ions. This reaction occurs due to the polar nature of water molecules, which interact with the acid molecules and cause them to break apart. The dissociation of an acid in water can be explained by the following steps:

1. Ionization of Acid

When an acid is added to water, it first undergoes ionization, where a proton (H+) is transferred from the acid molecule to a water molecule to form a hydronium ion (H3O+). This process is represented by the following equation:

HCl + H2O → H3O+ + Cl-

In this equation, HCl is the acid, H2O is the water molecule, and Cl- is the chloride ion formed after ionization.

2. Formation of Hydronium Ions

The formation of hydronium ions is a crucial step in the dissociation of an acid in water. Hydronium ions are formed by the combination of a proton (H+) and a water molecule (H2O). These ions are highly reactive and play an essential role in many chemical reactions.

3. Separation of Acid into Ions

Once the acid has been ionized and hydronium ions have formed, the acid molecule splits into its constituent ions. For example, in the case of hydrochloric acid (HCl), the acid molecule dissociates into a hydrogen ion (H+) and chloride ion (Cl-). This reaction is represented by the following equation:

HCl → H+ + Cl-

4. Formation of a Solution

The final step in the dissociation of an acid in water is the formation of a solution. As the acid dissociates into its constituent ions, the resulting solution becomes more acidic due to the presence of hydronium ions. The pH of the solution decreases, and it becomes more acidic.

In conclusion, when an acid is added to water, it undergoes a process known as dissociation, where it splits into its constituent ions. This process is crucial in many chemical reactions and is responsible for the acidic properties of many substances. The dissociation of an acid in water can be explained by the ionization of the acid, the formation of hydronium ions, the separation of the acid into ions, and the formation of a solution.