[HongKongALevelboy]

why is sodium hydrogencarbonate alkaline when dissolved in water

[jdurg]

I may not be 100% correct on this, but I believe it's because when dissolved in water, NaHCO₃ dissociates quite a bit into Na⁺ and HCO_{3^-.  The bicarbonate ion tends to grab some hydrogen ions from water and form carbonic acid.  Carbonic acid is a very weak acid, so it does not like dissociating back into the hydrogen ion and the bicarbonate ion.  As a result, there will wind up being a small bit of hydroxide ion in solution which will then make it slightly alkaline.  This is just what's on the top of my head right now, so it probably isn't 100% accurate.  Hopefully someone else will correct any errors I've made.}  

[movies]

jdurg is right on, as far as I can tell.  The Na+ and the bicarbonate should dissociate completely in aqueous solution, so there would be quite a lot of bicarbonate ions floating around to deprotonate water molecules.  The pK_a of carbonic acid is about 6 while the pK_a of water is about 16.  So, the equilibrium isn't very favorable, but it does shift enough to make the solution slightly alkaline.

Here is a more qualitative explanation in terms of Le Chattlier's principle:

Given the equilibrium
 
                          --->
HCO₃^- + H₂O    <---   H₂CO₃ + OH^-

Stressing the system by adding additional bicarbonate will cause the equilibrium to shift to relieve the stress, therefore shifting to the right, which produces more hydroxide.


I hope this all helps.