Final answer:
The pH remains stable when acids or bases are added to a buffer solution containing equal amounts of NH₃ and NH₄Cl, because the NH₃ and NH₄⁺ ions can neutralize added H⁺ or OH⁻ ions, thus resisting changes in pH.
Step-by-step explanation:
The reason the pH does not change significantly when a small amount of an acid or a base is added to a solution containing equal amounts of the base NH₃ and a salt of its conjugate acid NH₄Cl is due to the solution acting as a buffer. A buffer solution is a mixture of a weak acid or a weak base and the salt of its conjugate that resists changes in pH.
When an acid is added to this buffer, the NH₃ can act as a base and neutralize the added H⁺ ions, forming NH₄⁺. Conversely, if a base is introduced, NH₄⁺ can donate H⁺ ions to neutralize OH⁻ ions. This dual action of accepting or donating protons keeps the pH stable.