Originally Posted by
U.N. Owen
Biochemistry, but I do have some level of physics background because of my job.
Fire doesn't become plasma. In simplest terms, fire is a chemical reaction which gives off light and heat. It's less that fire becomes hot enough to turn into plasma and more that the gases around it are energized in such a way that they become plasma. Of course, this is a comic book.
I guess that Johnny burned hot enough to turn his physical body into plasma which opens up its own can of worms, but whatever, comic book science is comic book science.
Impervious to physical attacks? Not really. You probably have plasma in your house or seen it in your neighborhood through the form of neon signs or plasma televisions. They're contained. Plasma is just ionized gas at its basic level. Plasma can be contained like we see in those aforementioned signs and tvs. Hell, even Earth's ionosphere is plasma and it's contained in our atmosphere. Johnny really shouldn't walk through any solids like Kitty Pryde. He probably would melt through them given his plasma wouldn't be like those in our tvs and close to those in a star or lightning.