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Name: Michael
Status: educator
Grade: 6-8
Location: OR
Country: USA
Date: December 2008


Question:
We are talking about solutions as homogeneous mixtures. Is it correct to say that two characteristics of solutions are: uniform color and transparent? Can't a solution be translucent or opaque (solid)?



Replies:
Hi Micheal,

I answered a question about heterogeneous/homogeneous mixtures on NEWTON a few years back, so you may want to refer to that response as well as what I have written here.

The phrase "solution" is used everyday by chemists to describe a liquid homogeneous mixture. Most solutions are transparent. I actually cannot think of a true solution that is not transparent. Milk, for example, looks homogeneous but is really made up of microscopic particles suspended in an aqueous phase - actually, it is a colloidal suspension, and those all tend to be translucent or opaque in liquid form.

Probably all mixtures can be described as either homogeneous mixtures, colloidal suspensions, or heterogeneous mixtures. A lot of times in high school and freshman chemistry courses the possibility of a colloidal suspension is not even discussed.

Finally, there are such things as solid homogeneous mixtures. One example is an alloy. An alloy is a homogeneous solid mixture of two or more elements in nonstoichiometric proportions. An example of an alloy is brass (contains copper mixed with zinc), and brass is opaque, i.e., not transparent. Glasses can also be described as homogeneous solid mixtures, but they can of course be transparent.

Hope this helps!
Dr. Topper


A true solution in a non-metallic liquid like water can be absorbing (dark, black) to the point of opacity in the given container size, but if placed in a container that presents a thin-enough "slice" of liquid, it will become dimly transparent.

Translucence and non-black opacity both require scattering of light waves. To scatter E-M waves requires spatial variations in the index of refraction, variations over size ranges similar to or larger than the length of the wave being scattered. The wavelengths of visible light are on the order of 0.5 micrometer, which is a couple orders of magnitude larger than atoms or most molecules, larger even than the typical spacing between solute molecules in a dilute solution. And so the solution ends up being optically uniform and cannot do much scattering. This is the core logic of the assertion that true solutions are transparent. I think it is true, in a practical sense.

But it is not quite something one should mean as an absolute. For example, it does not quite mean zero scattering. Even pure air scatters a percentage of blue light in 10 miles of transit. Water too: you might not be able to clearly see a whale 5,000 feet down, even if the water was distilled, the surface optically flat, and the depths nicely illuminated. But it does tend to mean that water solutions in commonly experienced container sizes will look more clear than hazy.

Two orders of magnitude is not quite enough margin to make exceptions impossible. I suppose if one found a solute molecule with exceptionally high optical interaction strength,(i.e, a high dielectric susceptibility...) then a solution dilute enough to place molecules ~0.1 micron apart on average could end up cloudy.

Some large single molecule, spiked with one or more highly resonant groups, might do it, particularly for a narrow range of wavelengths near the resonance peak. Does that describe a dye molecule? Maybe. But most dyes are broadly resonant, and you would need to find one with a 10x or 100x narrower resonance. The transition-center would need to be unusually well-isolated from the environment around it. Building non-polar bulk around the center would help isolation, but would that be considered a separate phase? Maybe not, if it were part of the same covalently-bonded molecule. So we would have single-molecule latex balls with a resonant group in the middle and hydratable groups around the outside. Something plausible for a bio-molecule, but I do not actually know any examples.

A pretty large Bucky-ball would be good for permanently keeping out water molecules, but I think Bucky-balls usually have enough visible-range electron transitions of their own that our ball might electromagnetically couple to the resonant center we have placed inside, thereby making it a broader resonance, which therefore would have less spatial reach.

Some water-soluble polymer with large electron-delocalized groups might do it too. One would need to figure out if this molecule truly dispersed to separated individuals or clustered lightly but was hydrated along its whole length, or whether it made clusters that might be considered a separate phase. The case of cloudy solutions of "soluble" starch in water needs that kind of discrimination. I do not actually know much about its internal disposition.

There could be borderline cases of solutions which have non-uniform distributions of component species, but subtly enough that one does not quite consider the variations to represent distinct separate phases. These could be fairly cloudy, and with some absorbing dye they could be opaque and colored in largish containers.

There may be a practically-usable thermodynamic definition for true solutions.

If so, some optically scattering, partially phase-differentiated mix might mostly satisfy that definition. I suspect that such a cloudy mix would at some higher temperature fully disperse and become transparent.

If, over the temperature range of that transition, there is almost no change in latent heat or thermal capacity, then this mix would probably meet such a definition as a true solution.

Sorry if I sound stilted, but I wanted to show you why the proposition is more practical than theoretical, even though it is mostly true. It is roughly what you want to make of it.

In practice, I find the truism to be really useful, simply because to me it seems that any time a solution is not transparent, something distinct is happening that one wants to investigate and explain in order to know accurately the state of the mix.

As a truism for junior-high-school science, I like it. How to best hedge the assertion within the language a teacher uses, I have not thought through yet.

Jim Swenson


While it is convenient to classify materials as "soluble" or "insoluble", please do not push these classifications to the boundary limits.

Mixtures of insoluble materials can be opaque, translucent, or clear depending upon whether the index of refraction matches the "solvent", or whether the size of the dispersed, insoluble component is much smaller than the wavelength of visible light.

These various categories that we tend to place various substances need to be kept in perspective. They are only "general rules". There are always exceptions and/or limits of applicability.

Vince Calder



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