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alkali

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alkali

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)



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? Mentioned in ? References in classic literature
 
There are swift-flowing rivers which dash through jagged canons; and there are enormous plains, which in winter are white with snow, and in summer are grey with the saline alkali dust.
A Study In Scarlet by Doyle, Sir Arthur Conan View in context
He dragged you through an Arkansas post-oak swamp, let you dry for a moment on the alkali plains of his Idaho ranch, then whirled you into the society of Viennese archdukes.
The Four Million by Henry, O. View in context
And she saw Daisy thereafter, through days of alkali and heat, walking, stumbling, in the dust of the wagons, the little sick dog, like a baby, in her arms.
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