An acid according to the Arrhenius definition is a substance that, when dissolved in water, releases hydrogen ions (H+). These hydrogen ions are responsible for the acidic properties of the solution, such as its sour taste, ability to react with metals, and ability to turn blue litmus paper red.

This definition was proposed by the Swedish chemist Svante Arrhenius in the late 1800s and was a significant step forward in our understanding of acids and bases. It provided a more precise and measurable way of defining acids and laid the foundation for further developments in the field of acid-base chemistry.

Examples of acids according to the Arrhenius definition include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3). These acids, when dissolved in water, release H+ ions, leading to their characteristic acidic properties.

The Arrhenius definition, while groundbreaking at the time, has been expanded and refined over the years, particularly with the introduction of the Bronsted-Lowry definition and the Lewis definition of acids. These definitions provide more comprehensive explanations of acid-base behavior and are commonly used alongside the Arrhenius definition in modern chemistry.