The terms “base” and “alkali” are often used interchangeably, because most common bases are alkalis. It is common to speak of “measuring the alkalinity of soil” when what is actually meant is the measurement of the pH (base property). Similarly, bases which are not alkalis, such as ammonia, are sometimes erroneously referred to as alkaline.
Note that not all or even most salts formed by alkali metals are alkaline; this designation applies only to those salts which are basic.
While most electropositive metal oxides are basic, only the soluble alkali metal and alkaline earth metal oxides can be correctly called alkalis.
This definition of an alkali as a basic salt of an alkali metal or alkaline earth metal does appear to be the most common, based on dictionary definitions, however, conflicting definitions of the term alkali do exist. These include:
Any base that is water soluble Alkali, Farlex, 2008. Retrieved April 8, 2008. This is more accurately called an Arrhenius base.
The term “alkali” is derived from the Arabic word al qalīy, meaning “the calcined ashes.” These plant ashes were regarded as having properties such as the ability to reverse the action of acids and having detergent power. Thus, an alkali was initially thought of as the antithesis of an acid. The formation of salts from the acid and alkali reaction led to the view that salts can be derived from two constituents of opposite natures.
Yet, not all non-acidic constituents possessed alkaline properties. Examples are oxides and hydroxides of heavy metals. Hence, the concept of “base” was born. This concept was first introduced by the French chemist Guillaume François Rouelle in 1754. He noted that acids—which in those days were mostly volatile liquids, such as acetic acid—turned into solid salts only when combined with specific substances. These substances formed a concrete base for the salt, and hence
In chemistry, a base is thought of as a substance which can accept protons or any chemical compound that yields hydroxide ions (OH–) in solution. It is also commonly referred to as any substance that can react with an acid to decrease or neutralize its acidic properties, change the color of indicators (e.g. turn red litmus paper blue), feel slippery to the touch when in solution, taste bitter, react with acids to form salts, and promote certain chemical reactions (e.g. base catalysis). Example of simple bases are sodium hydroxide and ammonia. Sodium hydroxide (NaOH), also known as caustic soda or lye, dissociates in water to form hydroxide ions (OH–) and sodium ions (Na+).
Bases have many practical uses, and several of them are commonly found in the home. Household ammonia is a familiar cleaning agent. Lye is used for cleaning clogs and sink drains. Potassium hydroxide, also called caustic potash, is used to make soft soap that dissolves in water with ease. Magnesium hydroxide in water (also called milk of magnesia) is used as an antacid or laxative.
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.
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.