|MadSci Network: Chemistry|
Alan, 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. Reference: Physical Chemistry, 4th Ed., P. W. Atkins, W.H. Freeman and Company, New York, 1990
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