why is Na fusion necessary prior to the qualitative tests for elemnts other dan C,H and O in the unknwn result

Answer 1

To decompose the unknown, and convert the elements into a form that you can easily test for. For example, halogens are converted to halide, which give precipitates with silver nitrate,and nitrogen to cyanide, which gives the Prussian blue test.

If you left the elements covalently bound as they are in your unknown, they would not be free to react with your test reagents.