MadSci Network: Chemistry

Re: Why do substances in solution (aq) always become transperent?

Date: Thu Dec 2 15:42:04 1999
Posted By: Narayan Variankaval, Grad student, Polymers/Textile and Fiber Engineering, Georgia Tech
Area of science: Chemistry
ID: 943902365.Ch

     When you shine a light on a solution three things happen - a part of 
it is transmitted through the solution, a part of is absorbed and a part 
of it is scattered.  Scattering will occur if the size of particles in 
solution becomes larger than the wavelength of light (450 nanometers - 700 
nanometers). Depending on the extent of these three events you would see 
either a clear transparent solution, a completely opaque solution or 
something in between i.e. a cloudy solution.  In a clear solution, the 
amount of transmitted light is much greater than that of either scattered 
or absorbed light as in the case, for example, of water.  When more and 
more of a solute such as salt is added to it the concentration increases. 
As long as this concentration is lower than the solubility of salt in 
water the solution will remain clear. When this limit is exceeded, salt 
starts to precipitate out.  This process of precipitation is nothing but 
the aggregation of several particles of salt into bigger particles. As the 
size of these particles grow, more and more light is scattered by them and 
the amount of transmitted light reduces considerably. Now this will be the 
case if you keep stirring the solution and not allowing the particles to 
settle down. (When the particles start settling down the solution above 
them is still of a low concentration, below the solubility limit, and 
hence will be clear). 
	The above explanation is true for any solution, i.e. solution of a 
solute and solvent, not necessarily ionic in nature like salt. It works 
for sugar solutions too, which are primarily sucrose molecules and can 
hardly be called ionic. In all solutions, if each solute molecule (salt -
Na+Cl-, or sugar - sucrose) is completely surrounded by solvent molecules, 
a single phase completely mixed solution results. This solution will be 
completely transparent.  In the case of salt the solution will then 
contain sodium and chloride ions each of which is completely surrounded by 
water molecules; in the case of a clear sugar solution, each sucrose 
molecule (formula - C6H12O6), will be completely by water molecules.  
There are no particles larger than the wavelength of light (450-700nm). So 
light will not be scattered. Salt and sugar solutions (clear solutions) do 
not absorb in the visible region. Hence these solutions are colorless ( 
for a solution to have color it should absorb light in the visible 
region !!).  So all the light that shines on a salt or sugar solution is 
transmitted.  This makes them appear as clear solutions.  Now when the 
solubility limit is exceeded crystals of salt or sugar begin to form.  In 
a salt solution, Na+ and Cl- ions begin to come together. In a sucrose 
solution, crystals of sucrose begin to form.  These grow in size and at a 
certain concentration may exceed the wavelength of light.  Then scattering 
occurs and the solution starts turning cloudy. When a lot of salt or sugar 
is dumped in water, then the particles become so big that most of the 
incoming light is scattered and no transmission occurs.  This solution 
will look opaque.
	These phenomena of transmission, scattering and absorption are 
useful to scientists in millions of ways.  Some of them are 
(1) To determine the concentration of solutions if they are unknown
(2) To study the phase behavior of liquid mixtures
(3) To study the formation of aggregates in solution like micelles, 
surfactant molecules, detergents etc. so that the right formulation of 
soaps and other cleaning products can be designed
(4) To explain the more mundane phenomena such as why the sky is red in 
the mornings and evenings and blue most of the day
	I hope I have answered your question. I give below a reference 
that you might find useful if you have any questions.

Physical Chemistry, 4th Ed., P. W. Atkins, W.H. Freeman and Company, New 
York, 1990

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