Pure water at 25°C ionizes in the presence of acid to form an equilibrium in which ionizes in the presence of acid to form an equilibrium in which self-ionizes to form an equilibrium in which self-ionizes to form an equilibrium system in which

Pure water at 25°C indeed undergoes a process called self-ionization or autoionization, where water molecules react with each other to form hydronium ions (H3O+) and hydroxide ions (OH−). The equilibrium reaction can be written as:

2H2O(l) ⇌ H3O+(aq) + OH−(aq)

or for simplicity, it is often written as:

H2O(l) ⇌ H+(aq) + OH−(aq)

This reaction shows that a water molecule can donate a proton (H+) to another water molecule, which then forms the hydronium ion (H3O+), leaving behind an OH− ion.

At 25°C, the concentration of H3O+ ions and OH− ions in pure water is extremely low, about 1 x 10^-7 moles per liter each.

It is also important to note that the product of the concentrations of these ions is constant at a given temperature, known as the ion-product constant for water, Kw:

Kw = [H3O+][OH−] = 1 x 10^-14 at 25°C

Now, when an acid is added to water, it increases the concentration of H3O+ ions, which shifts the equilibrium to the left according to Le Chatelier's principle.

This means that less water will ionize since there are already more H3O+ ions provided by the acid.

Consequently, the concentration of OH− ions will decrease because the product of the ions' concentrations must still equal Kw.

The decrease in OH− concentration results in the solution becoming more acidic, which is why the pH falls when acid is added to water.