18. Solving a chemistry problem

7. Mix 3 drops of silver nitrate solution and 4-5 drops of potassium iodide solution in a test tube. Divide the resulting precipitate into 2 parts and place in 2 test tubes. Add an excess of ammonia solution to 1 test tube, and sodium thiosulfate solution to another test tube. Why does the precipitate not dissolve in the first test tube and dissolve in the second? Write the reaction equation.