When a small amount of hydrochloric acid (HCl) is added to a solution of Na₂HPO₄, the pH of the solution does not change markedly. The pH also does not change drastically when a small amount of sodium hydroxide (NaOH) is added to this same solution. Based on these observations, the compound Na₂HPO₄ is:

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The compound Na₂HPO₄ (sodium phosphate) is acting as a buffer because it minimizes changes in pH when small amounts of acid (HCl) or base (NaOH) are added. This is characteristic behavior of a buffer solution, which consists of a weak acid and its conjugate base or a weak base and its conjugate acid, helping to maintain a relatively stable pH in a solution.

When a Small Amount of Hydrochloric Acid (HCl) Is Added