Alkalis are a class of chemical compounds that are capable of absorbing hydrogen ions. Such as metal oxides or hydroxides are alkalis. Water-soluble alkalis that provide hydroxide ions ( OH−) are called alkalis according to the Bronsted-Laurie theory. Other theories or definitions of bases include electron pairing , source of hydroxide ions or Arhenius theory. These chemical compounds dissolve in water and release hydrogen ions, making the pH value of the solution higher than that of calm water, that is, more than. Notable among the most common bases are sodium hydroxide , ammonia. Chemically the opposite of acid is alkali. The reaction between acids and bases is called mitigation reaction. Acids and bases are considered opposite because acid increases the concentration of hydronium ions in water ; Alkalis on the other hand reduce it. Alkalis react with alkaline acids to produce salt and water. Alkalis which are soluble in water are called alkalis ; On the other hand, alkalis which are insoluble in water are alkalis but not alkalis. That is why it is said that not all alkalis are alkalis , but all alkalis are alkalis.