MadSci Network: Chemistry

Re: why benzene is colourless compound ?

Date: Sun Apr 18 12:18:26 2010
Posted By: Jerry Franzen, Chemistry Teacher
Area of science: Chemistry
ID: 1268408141.Ch

Materials are colored because they absorb some of the colored light of the visible spectrum of light. The colored light that is not absorbed is either transmitted by a transparent or translucent material or reflected by an opaque material.

A solution of a red dye will absorb all but the red light of the visible spectrum. The red light will be transmitted by the solution and a person's eye will see the red light and see the solution as being red. A blue painted surface will absorb all of the visible light but the blue part, which will be reflected from the surface. The eye will see this reflected blue light and see the object as blue.

The materials in dye, the pigment or any colored substance are able to absorb visible light because their molecules (or atoms in the case of some metals) have electronic energy levels whose differences in energy match the energy available in visible light. If the electronic energy levels are close enough together so their difference matches the energy of red light, the material will absorb light in the red portion of the spectrum and appear blue. If the energy levels are farther apart, their difference may match the energy of blue light. In this case the molecules will absorb the light in the blue region of the spectrum and the material will have a red color.

In benzene, the electronic energy levels are too far apart to match any part of the light in the visible spectrum, so benzene does not absorb any of the visible spectrum. All colors are transmitted or reflected by benzene and it appears as colorless. "White" or "colorless" light is actually light composed of all colors.

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