Re: Is steam more dangerous at 100 degrees C or water at 100 degrees C?

It's hard to answer a question like that in general, because "dangerous" 
depends on the context. There are specific situations where one or the 
other would be more "dangerous."

But the most common danger would be that of being burned when it comes 
into contact with your skin.  And there, steam is generally more 
dangerous. To understand why, you need to think about what happens when 
you get burned. The steam or water gives up energy to your skin (until it 
has given up enough to be cooled down to skin temperature), and if it is 
too much energy it causes damage. Now think about boiling a pan of water 
on a stove. You have to put in some energy to get it up to the boiling 
temperature. Then you have to put in a bunch more energy to get it to 
boil. So steam has a much higher energy content per gram than liquid water 
at the same temperature. All this energy is available to burn you above 
and beyond what the hot water would burn you, making steam more dangerous.

Now a little bit of a curve ball. The answer I just gave is based on 
dangers of steam in industry, where it is often used at fairly high 
pressures. So it would apply, for example, to the difference between 
pressurized steam and water both at 200 degrees C. You specifically asked 
about 100 degrees C. Boiling there is at atmospheric pressure, so the 
steam is pretty dilute. So even though each gram of steam would have much 
more burning potential than a gram of hot water, low-pressure steam is 
spread out enough that the amount of H2O that contacts you is relatively 
small. That's why you can hold your hand (at least briefly!) in the steam 
above a boiling pan of water when water from the pan splashing onto your 
hand would scald you. But if you're in a situation where the steam is 
fairly concentrated, like if it is high-pressure or if it is directed out 
a nozzle (such as when your teakettle whistles), then steam is quite 
dangerous.