Re: If you have 100% humidity and boil water what happens to the vapor?

Interesting question. 100% humidity is defined as the point when the air
is saturated with water - when it holds all of the water that it can hold.
That would seem to imply that you can't get anymore water into the air and 
that a heated pot of water shouldn't boil but that is not quite the case. 
Two things can happen.

The first is that the air can become "supersaturated". That is, it can hold
more than 100% water vapour. Not much but enough that you could boil water 
in air that is 100% humid. And yes, it is "impossible" to have more 
than 100% so the situation is unstable and is susceptible to collapse when 
there is a disturbance. The water vapour will settle out on a any available 
surface - be it a window pain or a mote of dust. You can actually see this
occur when you are boiling a kettle. The large white plume that emerges is
a result of supersaturating the air with water vapour.

The other thing that happens if you tried to boil water in 100% humid air 
is "rain". That is, the water vapour may immediately return to the liquid
phase and produce rain. This is what would happen if you get well above 
the supersaturation point and, indeed, this is why rain clouds form in the
first place. The maximum humidity (100% humidity) is dependent upon the
temperature of the air. As water vapour rises within the atmosphere, it 
encounters different temperature zones in the atmosphere. Supersaturation
and condensation occur resulting in water droplets forming around 
nucleation sites. Visibly, this process results in clouds forming. As more
water is condensed from the vapour phase to the liquid, the size of the
droplets can grow to the point where they will drop out of the cloud and
rain occurs.

Hope this answers your question.