[IanMc]

From this post:

http://btinterface.com/BTInterface/forum/viewtopic.php?f=9&t=140&p=498#p498

PCB = Printed Circuit Board, a fibreglass plate covered with copper on both sides.

Etching is the process of covering up the parts that you would want to remain as signal wires, usually with permanent marker type ink and then exposing the rest of the board to an acidic solution.

[Borek]

This was discussed here on several occasions, have you searched the forum? An some of these threads do answer your question.

http://www.chemicalforums.com/index.php?topic=50614.0

http://www.chemicalforums.com/index.php?topic=46593.0

http://www.chemicalforums.com/index.php?topic=26772.0

Most likely it is a mixture of oxygen and chlorine.