Acid and Bases
(1) Arrhenius concept: According to Arrhenius concept all substances which give H+ ions when dissolved in water are called acids while those which ionise in water to furnish OH– ions are called bases.
; 
Some acids and bases ionise almost completely in solutions and are called strong acids and bases. Others are dissociated to a limited extent in solutions and are termed weak acids and bases. 
, etc., are examples of strong acids and 
are strong bases. Every hydrogen compound cannot be regarded as an acid, e.g., 
is not an acid. Similarly, 
, etc., have 
groups but they are not bases.