Which of the following solution is a buffer? a solution containing hf and naoh, nh3 and nh4+, hcn and hno3?

The answer is NH3 an NH4+. Buffer solutions contain a weak base and its conjugate acid or a weak acid and its conjugate base. NH3 is a weak base and NH4+ is its conjugate acid.

Aphysical change is one that you can put in and remove like a ham sandwich. the stuff you put in there can be changem its not permanet. a chemical change is on that can't be change thats permanet. like when you make a cake and bake it. you can't get those ingredients back.

Fine out your self ponk