Can somebody explain if 3G stays on when you are on wifi on iPhone 4s?

[The Game 161]

Alot are saying 3G doesn't ever switch off unless are not in a 3G area but how does it work when on wifi? Does it just stay on? Surely having that on when you aren't using it is killing the battery?

 

[Jenja]

I'd guess it stays on because you make your calls and texts over 3G (unless you have a 2G signal).