An acid is a substance that donates protons (in the Brønsted-Lowry definition) or accepts a pair of valence electrons to form a bond (in the Lewis definition). A base is a substance that can accept protons or donate a pair of valence electrons to form a bond. Bases can be thought of as the chemical opposite of acids. A reaction between an acid and base is called a neutralization reaction. The strength of an acid refers to its ability or tendency to lose a proton; a strong acid is one that completely dissociates in water.
