What is an acid?

The correct answer defines an acid as a compound that liberates hydrogen ions (H⁺) when dissolved in water and has a pH level below 7. This is a fundamental concept in chemistry, where acids are substances that increase the concentration of hydrogen ions in solution, leading to an acidic environment. The pH scale ranges from 0 to 14, with values below 7 indicating acidity, 7 being neutral, and above 7 indicating basicity (alkalinity). The release of hydrogen ions is what characterizes a substance as an acid, contributing to its acidic properties and effects in chemical reactions.

In contrast, the other choices present definitions that do not match the characteristics of acids. For example, a compound that accepts hydrogen ions and has a pH above 7 corresponds to the definition of a base. A neutral compound with a pH of 7 describes substances like pure water, while a compound releasing hydroxide ions is also indicative of a base, further distancing it from the definition of an acid. Understanding these distinctions is crucial in the study of acid-base chemistry.