Ammonia and hydrogen chloride diffusion experiment

High School Chemistry
A classic demonstration experiment regularly carried out in High School Chemistry classes.
Equipment:
Glass tube and rubber bungs
Concentrated hydrochloric acid
Concentrated ammonia solution
Cotton wool and glass Pasteur pipettes
The glass tube was fixed horizontally using a standard laboratory clamp and stand.
Concentrated hydrochloric acid was dropped onto some cotton wool at one end of the tube and some concentrated ammonia was added to some cotton wool at the other end.
Rubber bungs were inserted into the open ends of the tube to prevent the chemcals escaping.
A white solid can be seen forming in the tube from about 1 minute 40 seconds on into the video clip.
Students are asked to explain the formation of the white solid, name it and explain why it forms where it does (initially).
The fume cupboard was turned off during this experiment and wisps of the same white 'smoke' can be seen outside of the tube more or less throughout the entire video clip. Students might be asked to explain this too.
The movie clip was shot using a smartphone but on playback on a Windows PC was found to be upside down. Therefore, the clip had to be rotated 180 degrees using Microsoft Movie Maker which also resulted in a smaller file size and a loss in quality of the original clip.