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Definition: alkali from Philip's Encyclopedia

Soluble base that reacts with an acid to form a salt and water. A solution of an alkali has a pH greater than 7. Alkali solutions are used as cleaning materials. Strong alkalis include the hydroxides of the alkali metals and ammonium hydroxide. The carbonates of these metals are weak alkalis.

Summary Article: alkali from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

In chemistry, a base that is soluble in water. Alkalis neutralize acids, and solutions of alkalis are soapy to the touch. The strength of an alkali is measured by its hydrogen-ion concentration, indicated by the pH value. They may be divided into strong and weak alkalis: a strong alkali (for example, potassium hydroxide, KOH) ionizes completely when dissolved in water, whereas a weak alkali (for example, ammonium hydroxide, NH4OH) exists in a partially ionized state in solution. All alkalis have a pH above 7.0.

The hydroxides of metals are alkalis. Those of sodium and potassium are corrosive; both were historically derived from the ashes of plants.

The four main alkalis are sodium hydroxide (caustic soda, NaOH); potassium hydroxide (caustic potash, KOH); calcium hydroxide (slaked lime or limewater, Ca(OH)2); and aqueous ammonia (NH3(aq)). Their solutions all contain the hydroxide ion OH, which gives them a characteristic set of properties.

With acids Alkalis react with acids to form a salt and water (neutralization). For example potassium hydroxide and nitric acid gives potassium nitrate and water (the ionic equation follows).

KOH + HNO3 → KNO3 + H2O

OH + H+ → H2O

With indicators They give a specific colour reaction with indicators; for example, litmus turns blue.

With ammonium salts Alkalis displace ammonia gas from ammonium salts.

NH4Cl + NaOH → NaCl + NH3 + H2O

NH4(s)+ + OH(aq) → NH3(g) + H2O(l)

With soluble salts Alkalis precipitate the insoluble hydroxides of most metals from soluble salts. For example iron chloride:

FeCl2 + 2NaOH → Fe(OH)2 + 2NaCl

Fe2+(aq) + 2OH(aq) → Fe(OH)2(s)


Acids, Bases, and Salts

Making ammonia in the laboratory and its uses

Reactions of bases and alkalis

pH scale

Titration method

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