MCAT General Chemistry Practice Exam 2026 – Comprehensive Test Prep

When a graph displays a quasi-horizontal line during a titration, it represents the buffer region of a weak acid-strong base titration. In this region, the pH of the solution changes very gradually with the addition of the titrant.

This behavior is characteristic of buffer solutions, which are capable of resisting changes in pH upon the addition of small amounts of an acid or base. In the case of a weak acid being titrated with a strong base, the weak acid partially dissociates, and the conjugate base formed can react with added H⁺ ions. As a result, there is a relatively stable pH until a significant amount of the titrant is added.

This gradual change in pH is not seen in strong acid-strong base titrations, where the pH changes more steeply, especially around the equivalence point. Furthermore, the equivalence point itself is typically represented on a titration curve as a vertical line where the pH rises sharply, rather than a quasi-horizontal line. If the graph suggested that no reaction was taking place, it would indicate a lack of change in pH while titrant is being added, which is not characteristic of the buffer region. Thus, the correct interpretation