Substances that give off hydroxide ions (OH–) when dissolved in water are known as bases. When a base is dissolved in water, it produces hydroxide ions, which are responsible for the basicity of the solution.
Common examples of bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2). When these substances dissolve in water, they dissociate into their constituent ions, releasing hydroxide ions into the solution. For instance, sodium hydroxide dissociates into sodium ions (Na+) and hydroxide ions (OH–).
The presence of hydroxide ions in a solution increases its pH level, indicating that it is basic. This is contrary to acids, which release hydrogen ions (H+) when dissolved in water and lower the pH.
In summary, if you dissolve a substance in water and it increases the concentration of hydroxide ions, that substance is classified as a base.