Copper tubes in air conditioners...quick questions..need ur help urgently..

KAS90

hey there..
I have an assignment where I have to choose a material and research about..
anyway, I chose to talk about copper tubes in air conditioners..My question is..
what is the function of this tube?
What is the most distinctive characteristic/property that makes manufacturers choose copper to be used in the manufacturing of these tubes?
Finally, I read that an amount of phosphorus(P) or silver(Ag) is added to the copper.. Why is phosphorus/silver added to the copper?
I appreciate ur help coz by answering these questions I'll understand so many things, and ill be able to finish my assignment..
Need ur help!! plz..!!
Thanks in advance..

KAS90

hey again..
no 1 has ne idea?
I came 2 know that brass is used in the air conditioners..but what is the tube or fitting the aircondiotioner used 4? what's it's function?
and is there a kind of test that tests the corrosive property of an alloy or element?
I really neeeed ur help..pllllz..
just 4 these 2 questions..4get about the one above!
thanx again in advance..