Question of the week

Due in discussion 11/10/2000

As we saw in class, Iodine (I₂) is yellow-brown (or orange-brown, depending on your personal boundary between orange and yellow) in solution by itself and blue-black in the presence of starch. The absorbance spectra of Iodine and Iodine plus starch are given in Figure 1 below. The curves show how much light is absorbed by each solution at all the wavelengths across the spectrum of visible light. (The absorbance values have been normalized, that is, they are expressed as percent of maximum absorbance, so that we can compare two solutions with different absolute absorbances. See p. 104 of the lab manual.)

1. Indicate at what wavelength each solution absorbs the most light, and what color that wavelength of light would be.

2. Indicate at what wavelength each absorbs the least light, and what color that wavelength of light would be.

3. Explain why each is the color it is, in terms of color mixing rules and color vision.

Figure 1. Absorbance spectra for Iodine in aqueous solution and in solution with starch.