Base Definition in Chemistry

Chemistry Glossary Definition of Base

Sodium hydroxide
Sodium hydroxide is an example of a base. Ben Mills / Wikimedia Commons

In chemistry, a base is a chemical species that donates electrons, accepts protons, or releases hydroxide (OH-) ions in aqueous solution. Bases display certain characteristic properties that can be used to help identify them. They tend to be slippery to the touch (e.g., soap), can taste bitter, react with acids to form salts, and catalyze certain reactions. Types of bases include Arrhenius base, Bronsted-Lowry base, and Lewis base. Examples of bases include alkali metal hydroxides, alkaline earth metal hydroxides, and soap.

Key Takeaways: Base Definition

  • A base is a substance that reacts with an acid in an acid-base reaction.
  • The mechanism through which a base works has been argued throughout history. Generally, a base either accepts a proton, releases a hydroxide anion when dissolved in water, or donates an electron.
  • Examples of bases include hydroxides and soap.

Word Origin

The word "base" came into use in 1717 by French chemist Louis Lémery. Lémery used the word as a synonym for Paracelsus' alchemical concept of a "matrix" in alchemy. Paracelsus proposed natural salts grew as a result of a universal acid mixing with a matrix.

While Lémery may have used the word "base" first, its modern usage is generally attributed to French chemist Guillaume-François Rouelle. Rouelle defined a neutral salt as the product of the union of an acid with another substance that acted as a "base" for the salt. Examples of Rouelle's bases included alkalis, metals, oils, or absorbent earth. In the 18th century, salts were solid crystals, while acids were liquids. So, it made sense to early chemists that the material that neutralized the acid somehow destroyed its "spirit" and allowed it to take solid form.

Properties of a Base

A base displays several characteristic properties:

  • Aqueous base solution or molten bases dissociate into ions and conduct electricity.
  • Strong bases and concentrated bases are caustic. They react vigorously with acids and organic matter.
  • Bases react in predictable ways with pH indicators. A base turns litmus paper blue, methyl orange yellow, and phenolphthalein pink. Bromothymol blue remains blue in the presence of a base.
  • A basic solution has a pH greater than 7.
  • Bases have a bitter flavor. (Don't taste them!)

Types of Bases

Bases may be categorized according to their degree of dissociation in water and reactivity.

  • A strong base completely dissociates into its ions in water or is a compound that can remove a proton (H+) from a very weak acid. Examples of strong bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH).
  • A weak base incompletely dissociates in water. Its aqueous solution includes both the weak base and its conjugate acid.
  • A superbase is even better at deprotonation than a strong base. These bases have very weak conjugate acids. Such bases are formed by mixing an alkali metal with its conjugate acid. A superbase cannot remain in aqueous solution because it is a stronger base than the hydroxide ion. An example of a superbase in sodium hydride (NaH). The strongest superbase is the ortho-diethynylbenzene dianion (C6H4(C2)2)2−.
  • A neutral base is one which forms a bond with a neutral acid such that the acid and base share an electron pair from the base.
  • A solid base is active in solid form. Examples include silicon dioxide (SiO2) and NaOH mounted on alumina. Solid bases may be used in anion exchange resins or for reactions with gaseous acids.

    Reaction Between an Acid and a Base

    An acid and a base react with each other in a neutralization reaction. In neutralization, an aqueous acid and aqueous base produce an aqueous solution of salt and water. If the salt is saturated or insoluble, then it may precipitate out of the solution.

    While it may seem like acids and bases are opposites, some species can act as either an acid or a base. In fact, some strong acids can act as bases.


    • Jensen, William B. (2006). "The origin of the term "base". The Journal of Chemical Education. 83 (8): 1130. doi:10.1021/ed083p1130
    • Johll, Matthew E. (2009). Investigating chemistry: a forensic science perspective (2nd ed.). New York: W. H. Freeman and Co. ISBN 1429209895.
    • Whitten, Kenneth W.; Peck, Larry; Davis, Raymond E.; Lockwood, Lisa; Stanley, George G. (2009). Chemistry (9th ed.). ISBN 0-495-39163-8.
    • Zumdahl, Steven; DeCoste, Donald (2013). Chemical Principles (7th ed.). Mary Finch.