Acids are defined as any chemical substance that increases the concentration of hydrogen ions (H⁺) in a solution. This concentration is often represented using brackets, such as [H⁺], indicating the amount of hydrogen ions present. 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 HCl are introduced into pure water, they will split into four hydrogen ions and four chloride ions. Consequently, the solution will have a higher concentration of hydrogen ions than it did before the addition of the acid. This increase in hydrogen ion concentration is crucial for various biological processes, which will be explored further in subsequent lessons. Understanding the behavior of acids lays the groundwork for further studies, including the exploration of bases in future discussions.