Explain why sodium hydrogencarbonate solution is alkaline.

HCO3- + H2O <> H2CO3 +OH- ---------(1)
HCO3- +H2O <> CO3 2- + H3O+----------(2)
HCO3- can act as bronsted base by accepting proton to from H2O to form H2CO3 and OH-

as alkaline solution involve OH- ion,thus HCO3- can act as a base.
As H2CO3 is more stable than CO3 2- due to intense of negetive charge of CO3 2-(that mean CO3 2- more negetive charge than H2CO3 while H2CO3 is neutral(less reactive)) ,the equilibirum of (1) lies on right more than (2),
thus HCO3- is tends to be alkaline rather than acidic.

extra information:
equilibrium constant of (1) =4.76 x 10^-11
equilibrium constant of (2) =2.4 * 10^-8

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extra information係for reference