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enter image description hereInstant coffee granules generally are of a light brown colour, while coffee made from it is a deep black. Pigments are far more concentrated in the dry form, to add water is to dilute them. Still, this results in a much deeper colour. Why is it?

enter image description here

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    $\begingroup$ Many solids are darker when wet than when dry. Might not be a matter of concentration, but some other physical effect regarding light scattering or something. I am sure some physicist can explain this. Also, consider than many things like foams (even coffee foam, actually) are lighter than the liquid they are made from, because they incorporate air. $\endgroup$ Commented Mar 18, 2022 at 18:43
  • $\begingroup$ How do you determine "darkness", and, more importantly, compare optical properties (Which ones? Are the lighting conditions the same?) of the liquid and solid (consider the amount of light reflected back at observer)? I suppose you would agree that there is a certain concentration threshold when you even subjectively would agree that the powder is darker than the diluted coffee. $\endgroup$
    – andselisk
    Commented Mar 18, 2022 at 18:44
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    $\begingroup$ @andselisk I think the observation is somewhat trivial. I will see to add two pictures anyway. $\endgroup$
    – HannesH
    Commented Mar 18, 2022 at 18:46
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    $\begingroup$ Also, please refrain from introducing new tags like coffee or melanoides, especially when it has a typo and refers to a snail instead of an actual chemical compound melanoidin. $\endgroup$
    – andselisk
    Commented Mar 18, 2022 at 18:54
  • $\begingroup$ @andselisk I thought the funktion to add new tags was there by intention. Was that as far off as the compound name? $\endgroup$
    – HannesH
    Commented Mar 18, 2022 at 19:20

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It's not actually a chemical reaction (so this question is more appropriate for Physics.SE, not Chemistry).

You can see exactly the same phenomenon when you spill water on your coloured shirt; the wet spot appears darker.

… The light blue cloth reflects a lot of the light that hits it. That's caused by the high index of refraction of the cloth. The reflected light of course can't get absorbed by the dye in the cloth. Water has an intermediate index of refraction, between air and many solid materials. That reduces the reflection, allowing more light to get to the dye where it's absorbed. The material looks darker. This seems particularly effective on porous materials, perhaps because much of the light is reflected at various angles several times before bouncing out. Water reduces the reflection at all the surfaces.

On my glossy white paint, the water doesn't matter since the paint surface reflects the light anyway. I think the black cloth has so many dye molecules within a wavelength or so of the surface that it absorbs almost all the light that hits it even without a water layer to help. (The boundary between the outside and inside of the material is in effect a bit fuzzy, just as the images of things seen in a light microscope are fuzzy by about a wavelength of light.)

Q & A: why are wet things darker? | Department of Physics | University of Illinois at Urbana-Champaign

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