Answer 1:
Molecules have a tendency to interact with
one-another, which causes them to stick to
one-another. Heating up molecules causes them
to vibrate (heat is a measure of
vibration), and
the more violently they vibrate, the more likely
they will break free of their
interactions.
Molecules of water, or anything
else, at any temperature, have a likelihood of
breaking away from interactions ('evaporating')
and a likelihood of re-attaching ('condensing').
The chance of a molecule evaporating rises
with temperature, and the chance of condensing
rises with its abundance in a gas. Thus,
liquids and solids maintain a 'vapor
pressure' of
the same substance all around them at all times, a
concentration of vapor such that the rate of
molecules condensing equals that of evaporation.
If, however, the temperature is high enough
that the vapor pressure is above the ambient
atmospheric pressure, then the vapor will expand
out into its surroundings (lowering its abundance
in doing so). This runaway evaporation is what we
call 'boiling'.
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