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Solution Characteristics
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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