A base is a chemical compound that releases hydroxyl ions (OH-) in an aqueous solution. Litmus paper in a base solution turns blue.
Bases in aqueous solutions dissociate to form hydroxide ions (OH-). For example, sodium hydroxide NaOH in water breaks down into sodium ions (Na+) and hydroxide ions:
NaOH → Na+ + OH-
Hydroxide ions give the solution its basic properties. They react with acids to form salt and water.
Litmus paper is used to determine the acidity or alkalinity of a solution. In a base solution, litmus paper turns blue because hydroxide ions shift the equilibrium of the indicator towards the formation of the blue form.
Thus, a base is an alkali-type substance that dissociates in aqueous solutions, releasing hydroxide ions and giving the solution an alkaline reaction.