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 (NH_3(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)