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Name: Leslie
Status: student
Grade: 9-12
Location: MD
Country: USA
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.

Vince Calder

Hi Leslie,

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)
Canisius College

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