Final answer:
A buffer solution is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid). It resists changes in pH when small amounts of acid or base are added. Examples include a solution of acetic acid and sodium acetate, and a solution of ammonia and ammonium chloride.
Step-by-step explanation:
A mixture of a weak acid and its conjugate base (or a mixture of a weak base and its conjugate acid) is called a buffer solution, or a buffer. Buffer solutions resist a change in pH when small amounts of a strong acid or a strong base are added. An example of a buffer that consists of a weak acid and its salt is a solution of acetic acid (CH₂COOH) and sodium acetate (CH3COONa). Another example of a buffer that consists of a weak base and its salt is a solution of ammonia (NH3) and ammonium chloride (NH4Cl).