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A Closer Look: Evaporation Versus Boiling
Are evaporation and boiling the same processes?
At first glance, it would
seem that evaporation and boiling might be the same process, because
both start with a liquid and end up with a gas.
However, there are some important macroscopic and microscopic differences
that help make the distinction.
What are the “macroscopic” differences?
Evaporation happens
only at the surface of a liquid and occurs at any temperature (so
long as the substance is a liquid at that temperature).
However, as
most people are aware, liquids evaporates faster at a higher temperature.
Boiling,
on the other hand, happens throughout the bulk of a liquid, usually starting
from some site on the inside of the container and
rising in a
bubble to the surface. It only happens when the temperature is above
the boiling point of that substance.
Boiling water.
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What are the microscopic differences?
Boiling occurs when the average motion
of particles is fast enough to overcome the forces holding them close
together. This happens
evenly throughout
a boiling liquid because the temperature is uniform throughout.
Evaporation
happens for the following two reasons:
- Not all particles in the liquid
are moving at the same speed and, as a result, the faster particles
are more likely to overcome
the
forces they
feel from their neighbors.
- The particles at the surface
of the liquid are only held in place by forces from the neighboring
particles beneath them,
whereas particles
in
the middle of the liquid have forces holding them on
all sides. Thus, particles at the surface find it easier to break away
from
the liquid.
In the cases of both boiling and evaporation,
the force between two particles is always present. The greater the space
between
the particles
becomes,
however, the weaker the force is between them. To break
the bond between two particles, one particle has to be moving
fast enough
to overcome
the pull of the other, until it gets so far away that pull
is diminished. An
analogy would be if you tried to jump off the Earth: we
cannot jump fast enough to get to where Earth's gravity has less
pull, but a
rocket can.
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