You can't answer this problem if context is not given. However, I tried to search a similar problem and I found this link: https://wenku.baidu.com/view/bf3fcb51f01dc281e53af0e2. Refer to that link to understand my answers.
1. The main reactant is Cl-. When reacted with Ag,
Ag+ + Cl- --> AgCl
Now, if CO₃²⁻ is present, it would also yield,
Ag⁺ + CO₃²⁻ --> Ag₂CO₃
It would yield silver carbonate, which is undersirable.
2. The decomposition undergoes through a process called photodecomposition. The reaction is
2 AgCl --> 2 Ag + Cl₂
The undesirable precipitate is Ag because it could co-precipitate with silver nitrate which should be avoided.
