silver nitrate test問題~~幫幫手plz.....

The purpose of adding excess nitric acid is to prevent formation of other white insolutble compounds which may affect the result.

because

AgCI + HNO3 ---> NO REACTION

while

these insoluble compound such as
AgCO3 and AgSO3 will dissolve in nitric acid

This can ensure that the white precipitate formed is AgCI
We can deduce that CI- is present in a solution.