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 means that when an acid dissolves in water, it releases H⁺ ions, and when a base dissolves, it releases OH⁻ ions. This theory was developed by Svante Arrhenius in the late 19th century and laid the groundwork for modern acid-base chemistry. It helps to explain many chemical reactions and is fundamental in understanding how substances interact in aqueous solutions.