Acids are defined as any chemical substance that increases the concentration of hydrogen ions (H⁺) in a solution. The concentration of hydrogen ions is often represented using brackets, such as [H⁺], indicating the amount of hydrogen ions present in the solution. A common example of an acid is hydrochloric acid (HCl). When HCl is added to water, it dissociates into two components: a hydrogen ion (H⁺) and a chloride ion (Cl^-). This dissociation results in an increased concentration of hydrogen ions in the solution.

For instance, if four molecules of hydrochloric acid are introduced into pure water, they will each split into one hydrogen ion and one chloride ion. Consequently, the solution will contain four hydrogen ions and four chloride ions, demonstrating a clear increase in the concentration of hydrogen ions compared to the initial state of the water. This increase in hydrogen ion concentration is crucial for various biological processes, which will be explored further in subsequent lessons.