In chemistry, an alkali (from Arabic: Al-Qaly القلي, القالي ) is a basic, ionic salt of an alkali metal or alkaline earth metal element. Alkalis are best known for being bases (compouns with pH greater than 7) that dissolve in water. The adjective alkaline is commonly used in English as a synonym for base, especially for soluble bases. This broad use of the term is likely to have come about because alkalis were the first bases known to obey the Arrhenius definition of a base and are still among the more common bases. Since Brønsted-Lowry acid-base theory, the term alkali in chemistry is normally restricted to those salts containing alkali and alkaline earth metal elements.