Answer. Base is defined as a compound that dissolves in water to give OH~ ions. Alkali is defined as a compound that is soluble in water and gives large number of QH~ ions. Strong bases are called alkalis like NaOH, KOH. But A1 (OH)3 is not alkali because it is weak base and in aqueous solution forms relatively small amount of OH~ ions. Some bases are soluble in water but some are not. For example NaOH is soluble in water but Be (OH) 2 is not .Hence, bases which completely ionize in water are called alkalis.