Physics Photo of the Week
a photograph of my raingauge that had been frozen during the night
of January 30, 2009. What caused the brown blob in the center of
the frozen rainwater?
Normally we think of rainwater as quite pure. However, birds have
scattered sunflower seed shells from the nearby birdfeeder. Some
of these dark sunflower seed shells were sitting in the bottom of the
raingauge. Some of the tannins, or dark pigments, dissolved in
the rainwater. As the stained water in the raingauge froze, the
freezing began on the outside of the cylinder. The freezing
freezes the water, but not the stains that were in solution. The
stains propagate inward remaining in the liquid water, and the
perimeter ice is much more pure than originally. This
process is called "zone refining". As a result the stains, still
in solution, propagate toward the center and become trapped as a
spherical region of dark material.
The photo at right shows the water in the raingauge after it was
allowed to thaw. The solution of the tannins has diffused in the
bottom region of the liquid.
This process of zone refining is analogous to purification by
distillation. Two compounds in a solution can be separated or
purified by boiling - one component boils at a lower temperature than
the other, so the vapor is much purer than the original solution.
In zone refining, the purification results from a difference in the
melting temperature of the two components.
Zone refining is also very technologically important in refining
ultrapure silicon to manufacture transistors, integrated circuits, and
computer chips. An ingot of Silicon is melted in one particular
location. The melt is propagated along the ingot by means of a
special furnace. With repeated passes of the melt along the
ingot, the impurities move with the melt, leaving a very pure ingot of
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to firstname.lastname@example.org.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
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