Question

Ammonia is strong base but c5h5n is border live base why

Answer 1

Bases are compounds which can accept a proton by donating a pair of electrons. When comparing strengths of bases there are a number of factors that should be considered - in short, the more 'stable' the base is, the weaker it is (lower tendency to pick up a proton).  

Ammonia has a greater tendency to pick up a proton than pyridine and the resultant NH4+ cation (conjugate acid of ammonia) can be stabilised by hydrogen bonding to water.  

The conjugate acid of pyridine (pyr.H+ (protonated on nitrogen)) is stablised by the electron releasing effect of the alkyl groups, but cannot form hydrogen bonds. Overall, this results in it being a weaker base.