What do weak acid solutions contain to maintain pH stability?

Weak acid solutions are able to maintain pH stability primarily due to the presence of both the weak acid and its conjugate base in equilibrium. This combination is known as a buffer system.

When a weak acid donates protons (H⁺ ions) to the solution, the conjugate base can react with any additional H⁺ ions that might be introduced, preventing a significant change in pH. Conversely, if the solution becomes too basic (i.e., loses protons), the weak acid can dissociate to release H⁺ ions and counteract the increase in pH. This dynamic equilibrium allows the solution to resist drastic changes in pH upon the addition of small amounts of acids or bases.

In contrast, other options imply the presence of components that do not play a supportive role in maintaining pH stability within weak acid solutions. Strong bases would significantly alter the pH rather than stabilize it, while only conjugate acids would not provide the defensive mechanism necessary against both acidic and basic inputs. Inert gases, on the other hand, do not participate in chemical reactions that influence pH. Therefore, the unique combination of weak acids and their conjugate bases is essential for effective buffer action and pH stability.