Thus, small changes in pH represent large changes in the concentrations of hydrogen ions. Pure water is neutral. It is neither acidic nor basic, and has a pH of 7.0. ... Acids are substances that provide hydrogen ions (H⁺) and lower pH, whereas bases provide hydroxide ions (OH^–) and raise pH.

How do hydroxide ions affect pH?

When a solution becomes a hundred times more acidic the pH will fall by two units. ... When a solution is neutral, the number of hydrogen ions equals the number of hydroxide ions. When the number of hydroxide ions is higher, the solution is basic. When the number of hydrogen ions is higher, the solution is acid.

Why does adding hydroxide ions increase pH?

As the concentration of hydrogen ions in a solution increase, the more acidic the solution becomes. As the level of hydroxide ions increases the more basic, or alkaline, the solution becomes.