Why does a white ring form when ammonia and hydrochloric acid diffuse?

Answer 1

Both ammonia and hydrochloric acid are colorless gases at standard temperature and pressure, but they are capable of reacting with each other to produce ammonium chloride, which is a white solid at standard temperature and pressure. Colorless gases are invisible to human eyes, but white solids are not. When the two gases diffuse toward one another, they form finely divided white solids that remain visible for a while as a white ring in the diffusion interface.

Answer 2

The white ring forms due to the reaction between ammonia and hydrochloric acid to produce a white solid compound called ammonium chloride. This compound precipitates out of solution at the interface of the two diffusing gases, creating the visible white ring.