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.
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
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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