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I am conducting an experiment on “sunset yellow (FCF)”.

Based on my observations, the substance, when in powder form, is bright red in colour (observed with naked eyes). When I dissolve the powder in distilled water, the solution turns dark red. And the increase in colour intensity is significant.

I am not sure about the reason behind this phenomenon. It is said that FCF can act as a pH indicator through showing different colours. I shall point out that the “water” is distilled and thus should be neutral in nature. My guess is that pH is not a factor leading to this.

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  • $\begingroup$ The solid is probably more concentrated than the solution, and then dilute solutions will often appear different in colour to concentrated ones. How we perceive colour is rather complicated which is why spectrophotometers (colorimeters) are normally used for this. $\endgroup$
    – porphyrin
    Dec 6, 2023 at 16:36

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Pictures might have helped.

The sunset yellow FCF molecule is actually a disodium disulfonate with an additional phenolic hydroxyl group and a diazo function, with overall formula $\ce{C_{16}H_{10}N2Na2O7S2}$. In neutral water solution we would not expect the sulfonic acid groups to be protonated or the phenolic function to be deprotonated, but their oxygen atoms may hydrogen-bond to the protic hydrogens of the water. Such hydrogen bonding may affect the light absorption from the extended conjugated systems to which all the oxygen atoms belong.

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    $\begingroup$ Sorry that I don’t have a photo with me. But that’s a nice suggestion for me to propose better questions in the future. Thanks for your detailed explanation. :) $\endgroup$ Dec 5, 2023 at 13:09
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    $\begingroup$ (+1) Especially for suggesting that “Pictures might have helped.” Practically everyone has a camera in their cellphone and there is no current legal requirement that said cameras can only be used to take “selfies”! ;-) $\endgroup$
    – Ed V
    Dec 5, 2023 at 13:48
  • $\begingroup$ Additionally, in the solid phase, there may be intramolecular interactions that change the absorption. Packing effects may also twist the molecule, leading to differences in the $\pi$-system. $\endgroup$
    – TAR86
    Dec 5, 2023 at 20:38
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Powders are often paler than large crystals of the same substance, or of the substance in solution, because of diffraction, interference and quantum interactions of light with small particles.

Some other examples:

  • $\ce{CuSO4·5H2O}$, is pale blue as a powder but deep blue as larger crystals. Start with the powder, grow crystals, and crush them to see that change.

  • Colloidal gold makes glass red ("cranberry" glass).

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  • $\begingroup$ That makes a lot of sense. Thanks! $\endgroup$ Dec 7, 2023 at 13:42

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