An alkali is a base that is soluble in water. Alkaline solutions contain hydroxide (OH-) ions, have a pH greater than 7, turn litmus paper blue, and are soapy to the touch. Strong alkalis include the hydroxides of the alkali metals, notably sodium hydroxide (caustic soda, NaOH), potassium hydroxide (caustic potash, KOH), and calcium hydroxide (slaked lime or limewater, Ca(OH)2). These strong (caustic) alkalis are corrosive and can cause severe burns. Common weak alkalis include ammonium hydroxide (NH4OH), sodium carbonate (Na2CO3), and potassium carbonate (K2CO3).
A strong alkali (pH 11 to pH 14) ionizes completely or almost completely when dissolved in water. By contrast, a weak alkali is only partially ionized. Alkalis neutralize acids to form salts. They have important industrial applications in the manufacture of glass, soap, paper, and textiles.