Acids and Bases

1. Autoionization of Water

It seems that water does not conduct an electrical current.

Further research with more sensitive equipment shows however that water is a conductor for electrical current, but in a very small extent. So even in pure water some ions are present, due to the autoionization of water.
2 H₂O(l) H₃O⁺(aq) + OH^-(aq)

This equilibrium reaction is mostly written in a more simple way as:

H₂O(l) H⁺(aq) + OH^-(aq)

The equilibrium constant K of this equilibrium is very small. So the equilibrium state is positioned completely at the left. It is called the ion-product constant or dissociation constant of water and the value is 10^-14 at 25°C:

As both ions (H₃O⁺ = hydroniumion / H⁺ = hydrogen ion or proton and OH^- = hydroxide ion) are formed simultaneously:

[H⁺] is not always equal to [OH^-]. There are three possible situations:

neutral solution

acidic solution

basic solution

Broomthymol Blue

acidic solution	yellow
neutral solution	green
basic solution	blue

Phenolphthalein

acidic solution	colorless
neutral solution	colorless
basic solution	pink

Litmus

acidic solution	red

basic solution	blue

Summary

Autoionization of water

H₂O(l) H⁺(aq) + OH^-(aq)

neutral solution

acidic solution

basic solution