I have boiled water over a bunsen burner in a paper cup in Science class. Water that boils is 212 degrees. Containers have a flash point higher than this. Even with this knowledge, I’ll prolly still boil my water in my packed stainless steel cook kit. hehe
this must be a canoeing technique
Don’t try this at home children
Liquids expand when they get hot. The water will be under pressure when you take the bottle out of the fire. Not super high pressure, but pressure.
It is very possible that the cap could lose its seal when you pick up the bottle, or more likely, the water will spew out when you open the cap, just like a shaken soda bottle.
Near boiling water spewing onto your hand is no fun.
Could be Even Worse
In theory, you could heat the water to well above the "normal" boiling point due to the build-up of pressure (which would be due more to the vapor pressure than expansion of the liquid). Heated and sealed in that way, it could remain hotter than 212 F for some time after removal from the fire (unlike non-pressurized liquids which cannot usually be made hotter than the boiling point). Then, when you crack open the top, presto, you return the contents to normal atmospheric pressure, causing the entire 212+F contents to instantly vaporize. It would be very neat to see this happen, but NOT at arm's length. When opened, a soda bottle of superheated water instantly would create an enormous hot cloud of steam, and you wouldn't want to be anywhere in the viscinity when that happened.
A hot dog can turn into a chicken frank
when plunged into boiling water.
I wouldn't use this except in the case of extreme emergency.... if left in the fire too long the bottle will fail, and the water will explosivly vaporize. It's called a BLEVE (Boiling Liquid Expanding Vapor Explosion).
Camping stores sell all sorts of great cookware that will work well for boiling water.
Bad plastic poison also
Plastic, especially cheap plastic, tends to leach chemicals into water. It would normally take a long time before the water has a chemical taste, but super heating the water might speed it up greatly. I would highly suggest that this is a bad idea for that reason, but obviously the posts above me make for a much more dangerous immediate concern as well.
Incidentally, the cleaner the water (or lower TDS) the more the plastic chemicals will leach into it.
And the bottle still could explode
Liquids contain dissolved gases. As water heats, its capacity to hold those gases diminishes, and the gases have to go somewhere.