Water Acid and Base in One

We saw earlier that water acts as a base when it reacts with HCl and as an acid when it reacts with NH3. 

Water is amphoteric it can act as either an acid or a base. Even in pure water, water acts as an acid and a base with itself, a process called self-ionization. 

In pure water, at 25 °C, the preceding reaction occurs only to a very small extent, resulting in equal and small concentrations of HsO and OH . 

So all samples of water contain some hydronium ions and some hydroxide ions. The product of the concentration of these two ions in aqueous solutions is called the ion product constant for water (Kvi)- 

The preceding equation holds true for all aqueous solutions at 25 °C. The concentration of times the concentration of OH will be 1.0 X 10 . In pure water, since H2O is the only somce of these ions, there is one H3O ion for every OH ion. Consequently, the concentrations of H30 and OH are equal. Such a solution is a neutral solution. 

---