Question

what happens when solid ammonium chloride is heated in a test tube

Answer 1

It actually decomposes in ammonia and hydrogen chloride. It’s possible to separate them, because ammonia is lighter, so if you heat it in an inclined tube, the ammonia will go up and the HCl will go down. You can detect bot gases with wet indicator papers put up and down the tube. Of course, when the gases cool toghether (if you don’t separate them), they recombine to form NH4Cl, and it appears that it sublimated.