Question #9ac80

2 Answers
Jun 13, 2017

Alkalis are water soluble bases.


Sodium hydroxide dissolved in water forms #NaOH(aq)# which is an alkali. Something like calcium hydroxide is not water soluble would be described as a base.

So both bases and alkalis react with acids and neutralise them, but only alkalis are water soluble. All alkalis are basic but not all bases are alkalis.

Jun 13, 2017

In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal chemical element. An alkali also can be defined as a base that dissolves in water.


Not all bases are alkalis. The most general definition is a Lewis Base, which is any compound that can accept a proton. #OH^-# ions do not necessarily need to be involved. In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons.

A Lewis acid is therefore any substance, such as the #H^+# ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor. A Lewis base is any substance, such as the #OH^-# ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor.

The Lewis acid-base theory explains why #BF_3# reacts with ammonia. #BF_3# is a trigonal-planar molecule because electrons can be found in only three places in the valence shell of the boron atom. As a result, the boron atom is #sp^2# hybridized, which leaves an empty #2pz# orbital on the boron atom. #BF_3# can therefore act as an electron-pair acceptor, or Lewis acid.

It can use the empty #2pz# orbital to pick up a pair of nonbonding electrons from a Lewis base to form a covalent bond. #BF_3# therefore reacts with Lewis bases such as #NH_3# to form acid-base complexes in which all of the atoms have a filled shell of valence electrons.

The Lewis acid-base theory can also be used to explain why nonmetal oxides such as #CO_2# dissolve in water to form acids, such as carbonic acid #H_2CO_3#.

#CO_2#(g) + #H_2O# -> #H_2CO_3#(aq)