Color Change During Phase Change
Date: Winter 2009-2010
What causes elements to change color during phase changes?
For example, iodine being a purple gas and a dark silverish solid.
Color changes with a phase change (commonly liquid to solid or vapor to
either condensed phase): This occurs because of the different optical
properties of the condensced compared to the gas phase. It would be
impossible (at least for me) to "explain" what causes all these optical
properties. There is a different "packing" of molecules that changes their
optical properties. There can be changes in the chemical composition under
certain conditions. An example of that is iodine monochloride, which has
properties similar to bromine. However, if the vapor is not condensed
carefully iodine trichloride forms (ICl3) which is a brilliant yellow color
against a violet background similar to iodine. What "causes" these color
changes is not predictable as far as I know.
Let us go back to some fundamental ideas about the color of
substances. If white light (containing all the visible colors, such
as sunlight) were to pass through a substance (such as a colored
liquid) and the light that manages to pass through is no longer
white but some combination of colors, then the colors that were
missing must have been interacted with the substance. That missing
light must have been absorbed, scattered, refracted, or converted to
heat. Either way, that missing light no longer reaches our eyes at
equal strength to the other colors that managed to pass through
without interacting with the liquid. So if, let us say, red and
yellow interacted with the liquid, then the liquid will appear
blue-green to us because those are the colors that managed to reach our eyes.
So when a substance changes color when it goes through a phase
transition, it must mean that the kind of interactions that the
substance had must have changed as the substance changed phase.
In the case of iodine, as a gas, the substance is interacting with
the light as a individual molecules (because gases have particles
that are far apart from each other). So this means that the iodine
molecule can absorb quite a bit of colored light, leaving only the
purplish colors for us to see. When the iodine becomes a solid, the
molecules of iodine are not close to each other in a crystal
structure. Now the light interacts with the arrangement of the
iodine molecules in the crystal. If the crystals are tightly packed,
they may act like a mirrored surface, reflecting and refracting the
light and can account for the silverish sheen.
Greg (Roberto Gregorius)
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Update: June 2012