What Is a Base in Chemistry? Definition and Examples

Base Definition and Examples
In chemistry, bases react with acids. They release hydroxide ions, donate electrons, or accept protons.

In chemistry, a base is a substance that reacts with acids to form a salt and which releases hydroxide ions, accepts protons, or donates electrons in aqueous solution. Learn about the properties of bases and see examples of bases and their uses.

Base Definition

All bases react with acids to form salts, but there are different definitions of what, exactly, a base is. Each base definition has a corresponding acid definition.

  • Arrhenius: An Arrhenius base releases hydroxide ions (OH) in aqueous solution. An Arrhenius acid releases hydrogen ions (H+) in aqueous solution. A base does not need to have hydroxide (OH) in its formula to be an Arrhenius base. For example, ammonia (NH3) reacts in water forming the ammonium ion (NH4+) and the hydroxide ion (OH).
  • BrønstedLowry: A Brønsted-Lowry base is a proton acceptor. An Arrhenius acid is a proton acceptor.
  • Lewis: A Lewis base is an electron pair donor. An Arrhenius acid is an electron pair acceptor.

Acids and bases may seem like opposite species in chemical reactions, but some substances can act as either an acid or a base. Such a species is said to be amphoteric. Water is a classic example, as it can act as a weak acid (donating a hydrogen ion or proton) or weak acid (donate OH or accepting a proton to form H3O+).

Strong and Weak Bases

A strong base is a compound that fully dissociates into its ions in aqueous solution. A weak base incompletely dissociates into its ions, so that the resulting aqueous solution contains weak base, its conjugate acid, and water.

Strong base: BOH + H2O  →  B+(aq)  +  OH(aq)

Weak base: BOH  +   H2O   ↔    B+(aq)   +   OH(aq)
Weak base: B  +  H2O   ↔    BH+(aq)   +  OH(aq)

The strong bases are classic Arrhenius bases made from alkali or alkaline earth metals and hydroxide ions.

Common Strong BaseFormula
barium hydroxideBa(OH)2
calcium hydroxideCa(OH)2
cesium hydroxideCsOH
lithium hydroxideLiOH
potassium hydroxideKOH
rubidium hydroxideRbOH
sodium hydroxideNaOH
strontium hydroxideSr(OH)2

Calcium hydroxide, strontium hydroxide, and barium hydroxide only fully dissociate in solutions with concentration values of 0.01M or lower.

Weak bases include the conjugate bases of acids and many other compounds, often containing hydrogen or nitrogen.

Common Weak BaseFormula
trimethyl ammoniaN(CH3)3
ammonium hydroxideNH4OH
sodium bicarbonateNaHCO3

Other Types of Bases

Other types of bases include superbases, neutral bases, and solid bases.

  • Superbase: A superbase is a Lewis base that deprotonates even better than a strong base. Superbases have very weak conjugate acids. They form by mixing an alkali metal (e.g., lithium, sodium) with its conjugate acid. Superbases don’t remain in aqueous solution because they are stronger bases than the hydroxide ion. A simple example of a superbase is sodium hydride (NaH). The strongest superbase is the ortho-diethynylbenzene dianion (C6H4(C2)2)2-.
  • Neutral base: A neutral base forms a bond with a neutral acid. The acid and base share an electron pair.
  • Solid base: A solid base acts as a base in solid form. Silicon dioxide (SiO2) and sodium hydroxide (NaOH) mounted on alumina are examples of solid bases. Solid bases find use in reactions with gaseous acids and in anion exchange resins.

Properties of Bases

Bases display several characteristic properties:

  • Bases taste bitter. (Don’t test this.)
  • They feel slippery or soapy. (Don’t test this.)
  • Basic solutions have pH values greater than 7.
  • A base turns litmus paper blue. It turns methyl orange yellow and phenolphthalein indicator pink. Bromothymol blue remains blue in the presence of a base.
  • Strong bases and concentrated weak bases are caustic. They react vigorously with acids and organic matter and can cause chemical burns.
  • Molten bases and aqueous bases are electrolytes. They conduct electricity.
  • Bases react with acids to form salt and water.

10 Examples of Bases and Their Uses

Here are 10 examples of bases, their formulas, and their uses.

sodium hydroxideNaOHMaking soap, detergent, paper; drain cleaner; refining petroleum
potassium hydroxideKOHMaking soap; battery electrolyte
calcium hydroxideCa(OH)2Making plaster; leather production
magnesium hydroxideMg(OH)2Laxative; antacid
ammoniaNH3Making nylon, nitric acid, fertilizers; cleaning agent
aluminum hydroxideAl(OH)3Antacid; deodorant
methylamineCH3NH2Making drugs, insecticides, paint removers, surfactants
pyridineC5H5NAlcohol denaturant; solvent; making dyes, drugs, rubber products, vitamins
zinc hydroxideZn(OH)2Absorbent in surgical dressings; making pesticides and pigments
lithium hydroxideLiOHMaking lubricating grease and rebreathers

Reaction Between an Acid and a Base

An acid and base react in a neutralization reaction that forms a salt and water. The salt may dissociate into its ions or, if it insoluble or saturated, may precipitate out of solution as a solid.


  • 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.

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