An acid–base reaction is a fundamental chemical process involving the reaction between an acidic substance and a basic substance. In chemical theory, an acid is defined by its capacity to donate hydrogen ions ($H^+$), while a base is characterized by its ability to accept these ions or neutralize acidity. When these two classes of compounds react, the transfer of protons typically results in the formation of a salt and water. This type of reaction is critically important in various fields, serving as the chemical basis for pH measurement and neutralization processes. In biological systems, acid–base balance is maintained through complex buffering systems, ensuring that physiological fluids remain within narrow, stable pH ranges necessary for life processes.