Question

why are some salt solutions acidic? and why are some basic?

Answer 1

The answer to your question is the process called hydrolysis. The salt dissociates into two parts. One part reacts with free OH- in the water and the other reacts with H+. The new compounds that form have different tendencies to dissoociate, and you can end up with either more H+ than OH- (an acid), or vice versa (a base).

See
http://dbhs.wvusd.k12.ca.us/webdocs/AcidBase/Hydrolysis.html
for more details.