368 



Water, it will be remembered, is ionized to a very small extent. 

 When any salt is dissolved in water, therefore, there exists the 

 possibility of a double decomposition taking place. Thus with 

 sodium chloride: 



NaCl ^ Na+ + Cl~ 

 H 2 O ^OH- + H+ 



1! 11 



NaOH HC1 



In this case, however, the extent of formation of NaOH and HC1 

 is altogether negligible. NaOH and HC1 are both very highly 

 ionized in aqueous solution, their existence in appreciable amount 

 would involve the presence of both OH~ and H + in quantity, and 

 these ions would immediately combine to form water. 



When a salt like sodium carbonate is dissolved in water, the 

 state of affairs is different, as may be seen by studying the arrows 

 in the ionic equations: 



Na*C0 3 ^ 2Na+ + CO 3 = 

 2H 2 ^ 20H- + 2H+ 



11 1t 



2NaOH H 2 C0 3 



Double decomposition with the ions of water here involves the 

 formation of some NaOH and some H 2 C0 3 . Now the latter sub- 

 stance is an exceedingly weak acid. While, therefore, any NaOH 

 formed is almost entirely ionized, furnishing OH~ to the solution, 

 any H 2 C0 3 formed remains, on the contrary, almost entirely 

 in the non-ionized state. Now we have learnt (p. 241) that one 

 way of driving a reversible reaction to completion is to remove 

 one product as a non-ionized substance. This reaction cannot 

 be driven to completion in this way, since we have the formation 

 of another practically non-ionized substance, water, tending to 

 drive the reverse reaction to completion. In such circumstances, 

 obviously, a balance must be struck between the two conflict- 

 ing tendencies, and we will be left with a solution in which partial 



