You don't, you cut it and start over.
Usually toilet flange is glued down onto plastic drain pipe. There is a toilet flange that can be glued inside of plastic drain pipe also.
Copper pipe is soldered. Steel pipe is welded. Plastic pipe (pvc) is glued. There is even a type of pipe that is smoked...
Even if they are glued with ABS yellow glue, they will come apart. I have sometimes removed old glued in pipe from couplings by working a small screwdriver in between, then a bigger screw driver, then one side of long nose pliers and twisted. ( I do this only if the alternative is destroying a complicated section of pipe.)
It depends on the thickness of the plastic pipe. :)
To separate bricks that have been glued with Liquid Nails you will need Windshield Cut-Out wire. Begin pushing the wire between the bricks at one corner. Continue using the wire to separate the bricks.
Plastic is made from oil, usually fossil fuel oil.
Cut the pipe and remove it, fix pipe with a coupling.
There are different types of 'cap' that are plastic and glued to the drainpipe. If this is under the kitchen sink, and is a 'p' trap, then the plastic nut either side of it will unscrew with a pair of channel lock pliers or a small pipe wrench. If, on the other hand this is a 'cleanout' cap on the end of a drainpipe, then there will be a 'nut' formed into the end of it. Turn this nut with a wrench to remove it. Have shallow bowl or small bucket ready to catch water from this.
Yes, plastic pipe, PEX, can replace copper anywhere now.
Orange pipe is for natural gas.
Thomas S. Walsh has written: 'Plastic pipe and fittings' -- subject(s): Plastic Pipe, Pipe fittings, Standards
Yes, this is often done in well systems by using a mechanical fitting on both. ie, a fitting glued to the PVC, that can be threaded onto the galv. pipe.