Molecules have a tendency to interact with
one-another, which causes them to stick to
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
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.
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
Your question why does water boil from heat is an
What heating water really
does is provide thermal energy. When water gains
enough thermal energy it changes phases, from
liquid to vapor. When it changes into the vapor
phase, it forms pockets of water vapor in the
liquid.Because of the differences of density
between the liquid and vapor the vapor rises as
bubbles. The movement of the bubbles then
displaces the water. When enough of this is
occurring you get boiling.
Why the water
changes phases, I can only answer, that just sort
of is the way the universe works.
Hope that helps.
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