It is a universal rule that when solutes dissolve in solids they become translucent

What if you have a solute that is colored something like dark brown or black, if it dissolves in the solution will the solution still be translucent?

Is a non-translucent mixture a suspension? Or does translucence not even play a role in this stuff?

That should say

‘It is a universal rule that when solutes dissolve in solvents they become translucent’

If that were the case, then all water-based liquids would be clear. As a quick look through your refridgerator will show you, they aren’t.

I’m not sure I understand the question. If a solid dissolves in the liquid, the liquid would normally be transparent, not translucent. Just dissolve sugar or salt into water and you’ll see.

If a solid does not dissolve but is held in suspension, then it would be translucent because light scatters off the particles’ surfaces.

Yes, all true solutions are transparent. Anything liquid that is not transparent, indicates that scattering is taking place which is a result of a phase separation. Milk, for example, is not a true solution. Instead it is a suspension of colloidal particles.

Of course if what you are dissolving has a high absorption coefficient (such as a dye) then the solution would technically be transparent, but look completely black. I guess I’m not sure if I could call that transparent.

Mercury?

Mercury. Hmmm

Mercury clearly does not obey that rule. It’s a metal, so optically it behaves like a metal. I wonder if there are other liquids that don’t obey this rule?

Millions I would think. Take almost any solid and melt it and it will not be transparent. Lava isn’t transparent for example and of course all molten metals are just as opaque as mercury.

In fact the general rule only seems to apply if the liquid remains transparent in its solid state. Ice is transparent, so is water. Iron is opaque and so is molten iron. Salt and sugar are transparent and remain so in solution. Copper sulphate isn’t transparent and the solution is also quite clearly blue rather than transparent. Most rock isn’t transparent and the resultant lava isn’t either. Sand can be transparent and the resultant molten form is also transparent.

It’s true that molten metals do not obey this rule. I had thought that this was understood. I have a chunk of molten potassium in the still next to me and it looks like a glob of mercury. Definitely not transparent.
I am not certain that lava would qualify as a true solution. For one thing, I would be willing to bet that there were phase boundaries in molten lava. I am not a vulcanologist. Additionally, lava is so hot that it radiates on its own. This hides whether or not lava is transparent. I am quite certain that if lava is a true solution, it obeys this law.

  Copper Sulfate solutions most definitely qualify as transparent.  Transparent is not the same as colorless.  Transparent simply defines something that can be seen through with clarity.

 I think there needs to be some understanding of what this "Universal law" is implying.  As I mentioned as concentrated solution of black dye would be so dark as to absorb all of the light that passes through.  Nevertheless, it still obeys the law because this solution does not exhibit scattering.  Scattering is the result of phase boundaries that are a result of a liquid that is not uniformly mixed.

So here are the two exceptions:

1- dark solutions (They obey the meaning of the law, but probably don’t qualify as transparent.)

2- liquid metals (Perhaps they are transparent at wavelengths that aren’t reflective.)

I forgot to mention liquid crystals. Liquid crystals aren’t true liquids so technically they don’t count. If you have ever seen a liquid crystal, you know that they are cloudy or even milky. That is because there are phase boundaries in a liquid crystal.

Can a liquid metal actually be a solvent? Is the term solvent valid for such a liquid?

When gold ‘dissolves’ in mercury is it correct to call them solute and solvent?

Yes, I would say that a liquid metal can be a solvent. I don’t know why any liquid couldn’t be a solvent although the mechanisms of solvation can very greatly.

Well there is polar and non-polar solutions, but is it valid to call gold in mercury which is an amalgam or alloy of mercury a solution. Wiki suggests that it is correct http://en.wikipedia.org/wiki/Amalgam but just double checking here.
Would gold-mercury alloy be classed as a non-polar solution?

The concept that you are looking for is called Mie scattering. When the wavelength of light is large compared to the size of a scattering particle, the medium is transparent. This explains why individual molecules (~1 nm) dissolved in solution allow visible light (400-800 nm) to pass through, as well as why radio signals (1 m wavelength) can pass through larger objects (such as concrete walls).

When you have a solid, the space between the molecules diminishes, and light can no longer pass through.

I don’t seem to understand the idea “All true solutions are transparent … except the ones that aren’t.”

I can get the idea that some solutions are not transparent for other reasons than non-solutions are, but that doesn’t make them any less transparent, right?

To me it sounds like saying, “polar bears aren’t white” when in fact they are white, just not white due their fur being white, the way an elephant is grey.