The Arrhenius Theory is a scientific explanation of how acids and bases behave in water. According to this theory, an acid is a substance that increases the concentration of hydrogen ions (H⁺) in a solution, while a base increases the concentration of hydroxide ions (OH⁻). This theory helps to understand the properties and reactions of acids and bases in aqueous solutions. Developed by Svante Arrhenius in the late 19th century, this theory laid the groundwork for modern acid-base chemistry. It provides a simple way to classify substances as acids or bases based on their ionization in water, making it easier to predict their chemical behavior.