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
[14 century] English acquired alkali via Latin from Arabic al-qalīy ‘the ashes’, a derivative of the verb qalay ‘fry’. The implicit...
A substance which, on dissolving in water, splits up to a greater or lesser extent into ions and results in an excess of hydroxyl ions over...
chemical compound with formula KOH. Pure potassium hydroxide forms white, deliquescent crystals. For commercial and laboratory use it is usually in