4
$\begingroup$

If you can use precipitate colours to determine an anion, what makes the precipitate have that particular colour?

Like for example what in sodium sulfate or barium chloride makes a white precipitate when these two combine?

$\endgroup$
  • $\begingroup$ White is the default color. You don't have to do anything to produce it. $\endgroup$ – Ivan Neretin Aug 3 '17 at 5:19
  • 2
    $\begingroup$ Possible duplicate of Why is snow white? $\endgroup$ – Mithoron Aug 3 '17 at 19:05
5
$\begingroup$

The 'white colour' is due to the fact that the product does not absorb light in the visible part of the spectrum but forms as many fine crystals which in themselves scatter room lights. So the 'whiteness' is due to the scattering of light back towards your eyes after the light is repeatedly reflected of the many facets of the many crystals in the ppt. If you use red light to illuminate then the 'white colour' will become red. If the ppt is coloured then absorption of light occurs at some wavelengths as well as scattering at all wavelengths.

At the surface of a crystal some small fraction of light is reflected, but with millions of randomly oriented crystals numerous reflections occur. The amount of reflection at each surface depends on the difference in refractive index between the crystal and air for the wavelength of light used. If you could add a solvent say dichloromethane which has a high refractive index and will probably not dissolve the ppt then the crystals can be made to almost disappear as scattering is now far less. You can do the same thing with frosted glass beads which, when in air, scatter a lot but in water become virtually invisible.

$\endgroup$
-3
$\begingroup$

The following answer applies to all questions regarding color.

Color is the result of a substance absorbing certain wavelengths of light and reflecting others.

Different molecules have different shapes (geometry). Different shapes absorb/reflect different wavelengths. In addition to the shapes, the other parameter regarding color is the temperature and pressure.

Closely packed molecules will absorb/reflect light different from loosely packed ones.

2 more things that can "produce colour" rather than reflection of photons hitting it, are:

photons generated from chemical reactions and photons generated from radiation

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.