Base, in chemistry, commonly refers to any substance that can react with an acid to decrease or neutralize its acidic properties. A base is also called an alkali. When dissolved in water, bases feel slippery and taste bitter. Solutions of bases turn red litmus paper blue.
Chemists often define a base as any chemical compound that yields hydroxide ions (OH-) when dissolved in water. For example, sodium hydroxide (NaOH), also known as caustic soda or lye, is a base that dissociates (breaks up) in water to form hydroxide ions and sodium ions (Na+). The strength of such a base depends on the degree of dissociation. A strong base dissociates almost completely into ions. A weak base forms only a few ions. Strong bases, such as lye, can cause serious burns.
A base also may be defined as a chemical substance that readily combines with a proton. However, a base is most broadly defined as a substance that provides a pair of electrons to form a chemical bond. Under this definition, some chemicals that do not contain hydroxide ions are also classified as bases.
Bases have many practical uses. For example, many household drain cleaners contain sodium hydroxide, which can break down grease. Potassium hydroxide (KOH), also called caustic potash, is used in making soft soaps that dissolve easily in water. Magnesium hydroxide (Mg(OH)2) is often used as an ingredient in antacids. It is also the main ingredient in milk of magnesia, a common laxative.
