A base is a compound composed of metal cations and hydroxide ions. In the acid-base ionization theory, a base refers to a substance whose anions ionized in an aqueous solution are all OH-; in the acid-base proton theory, a base refers to a substance that can accept protons; in the acid-base electron theory, a base refers to an electron donor. .

Classification of bases

1. According to the number of hydroxide ions ionized by an alkali molecule: monobasic, dibasic, polybasic ;

2. According to solubility: soluble alkali, slightly soluble alkali, insoluble alkali;

3. According to ionization ability: strong base, weak base.