Substances that release hydrogen ions when dissolved in water are called acids, while those that release hydroxide ions are called bases. The relative concentrations of hydrogen and hydroxide ions in a solution determines how acidic or basic the solution is.
The pH scale is a measure of acidity and runs from 0 to 14. A solution with:
- Almost identical numbers of hydrogen and hydroxide ions is neither acidic nor basic, has a pH of 7 and is considered neutral.
- More hydrogen than hydroxide ions has a pH less than 7 and is acidic.
- More hydroxide than hydrogen ions has a pH greater than 7 and is basic.
When an acid is added to a basic solution, the hydrogen and hydroxide ions react to form water, and the pH of the solution changes.