I'm not sure what it is, so I'll call it skim. But after microwaving a can of heinz chicken soup, it always leaves ring of slightly thicker layer of soup on the inner side of the bowl just above the surface of the soup. Why does this happen?

It's the bit that gets the hottest and is drying out, turning gloopy. Just a guess, but it sounds good!

I dislike when that happens. I had to throw out another microwave bowl as it had a horrible ring where the soup or liquid normal reaches.

It is like it actually eats into the plastic.

What plastic?
A plastic bowl in a microwave?

I usually microwave mine in the bowl I'm going to eat it out of. Less washing up that way, plus it doesn't ruin them.

Best way to prevent or at least minimise this is to take it out and give it a good stir half way through the heating/cooking process.

This, plus cover in clingfilm and spike a couple of hole in it.

http://direct.asda.com/george/george...ap&istBid=tztx