Alkalis are a class of chemical compounds that are capable of absorbing hydrogen ions. Such as metal oxides or hydrochmides are alkalis. The water-soluble alkali, which is made up of hydrochmide ions, is called alkali according to the Bronsted-Laurie doctrine. According to other doctrines of alkalis or conjunctions include electron pair donation , source of hydrochmide ions or Arhenius doctrine. Chemically the opposite of acid is alkali. The reaction between acids and bases is called promotion reaction. Salt and water are produced by the reaction reaction with alkaline acids. Alkalis which are soluble in water are called alkalis whereas alkalis which are insoluble in water are called alkalis but not alkali. That is why it is said that all alkalis are alkalis but not all alkalis are alkalis. References Wikipedia.