Regarding the colour of gold sol, my book says

Finest gold sol is red in colour; as the size of particles increases, it appears purple, then blue and finally golden.

This is the image I found on Wikipedia which portrays the same phenomenon.

enter image description here

The same page also discusses the effect of size etc. on the colour, but I don't understand exactly what's happening.

Why shouldn't the colour be the same for all sizes? Isn't it characteristic of the molecules which make the sol as the light absorbed and reflected depends on them?

Does this mean that when we break down any other thing to very small particles, it's sol will change in colour?

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    $\begingroup$ Generally, the electronic structure of a small particle is a function of size - the high fraction of atoms that are on the surface alters the overall bonding. So, yes, changing size can change the color you see since the electronic transitions in the small volume are different. $\endgroup$ – Jon Custer Mar 10 '19 at 0:47
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    $\begingroup$ Yes, the color of colloids is dependent on size. Besides absorption, there is scattering as well. @Natasha, read about the term plasmon, which is collective oscillation of charge density in an electrical conductor. This is the beauty of nanochemistry although Faraday the great knew about this property long time ago. $\endgroup$ – M. Farooq Mar 10 '19 at 1:18

The color of conductive nanoparticles depends on plasmonics. In brief, quantum behavior predominates in the few electrons of a nanoparticle, rather than as a continuous conduction band. Surface plasmons strongly influence the color and polarization of light in sols and colloids. For example, for hundreds of years, stained glass has been made with gold or selenium to get pinks and reds.

  • $\begingroup$ Does it mean that if we can create nanoparticles of something else, it'll behave in the same way? Or is it characteristic to gold? $\endgroup$ – Eagle Mar 10 '19 at 7:26
  • $\begingroup$ @Natasha Not for, eg, a covalent crystal. Take sugar or carotene, in this case the predominant transition are those of the molecules. Those effects are evident in solid with extended bonding such as metals. Be careful to isolate various mechanisms. The final colour of the sample can rise from abs and various scattering mechanisms. The point is Au nanoparticles can be loosely seen as "molecules". Look what happens at the gold levels when you combine an increasing number of gold atoms. One gets a reduction of the en. gap and a shift in absorption. $\endgroup$ – Alchimista Mar 10 '19 at 9:51
  • $\begingroup$ @Natasha Yes. There are many nanoparticle systems where the behaviour is dominated by the size of the particle. Semiconductor nanoparticles can be tuned to particular colours by size, for example. $\endgroup$ – matt_black Mar 10 '19 at 13:15

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