What defines an acid salt?

An acid salt is defined as a salt that arises from the partial neutralization of a polyprotic acid, resulting in a compound that contains both acidic and basic properties. This typically occurs when a weak base combines with a strong acid.

In this case, the weak base does not fully neutralize the strong acid, which leads to a salt that can still donate protons (H⁺ ions) in solution, utilizing the acidic characteristics of the remaining hydrogen ions. This is why a weak base in combination with a strong acid is fundamental to the formation of an acid salt, allowing the salt to exhibit the properties of both an acid and a salt.

Understanding this concept clarifies why the other options do not qualify as defining an acid salt. For instance, a strong base combined with a weak acid would lead to a regular salt but lacks the acidic characteristics necessary for an acid salt since the weak acid would be completely neutralized. Similarly, strong base with strong acid, or weak base with weak acid would not be effective in producing a substance that exhibits both acidic and basic behaviors prominently.