What is an acid?

An acid is best defined as a substance that donates hydrogen ions (H+) in a solution. This fundamental property stems from the Arrhenius and Brønsted-Lowry definitions of acids. In the context of water, when an acid dissolves, it releases H+ ions into the solution, increasing the concentration of these ions. This release of H+ is what characterizes a substance as an acid.

When we look at the other options: a substance that accepts hydrogen ions (H+) describes a base, not an acid. The definition of an acid is focused on the donation of protons rather than their acceptance. While acids can indeed neutralize bases through a chemical reaction, this capability does not define what an acid is; it's an application of their properties in a chemical reaction. Finally, substances with a pH greater than 7 are considered basic, not acidic; acids generally have a pH less than 7, reinforcing the definition that aligns with the behavior of donating H+ ions.