When an acid and a base react with each other, the reactants
and products are in equilibrium with each other. Note the two-way
arrows. They indicate that this is an equilibrium reaction. That is, the
reactants on the left side of the equation are reacting and forming
product, and the products on the right side are also reacting and
forming the starting reactants. Chemists call the acid and base on the
right side of the equation the conjugate acid and conjugate base.
The reaction below is labeled to show the conjugate acid and conjugate
base.