MadSci Network: Chemistry

Re: What is the difference between a solution, a suspension, and a colloid?

Date: Thu Sep 30 02:03:48 1999
Posted By: Werner Sieber, Research Scientist, Pigment Division, Ciba Specialty Chemicals Corp.
Area of science: Chemistry
ID: 938606258.Ch

Dear Mary,
Suspensions, colloids and solutions are all different kinds of "mixtures" 
comprising at least two components, let's call them A and B. The difference 
lies in the SIZE (diameter, extension) of the particles of A surrounded by B. 

In the case of suspensions AND colloids, A is generally a solid and B is a 
liquid.  The solid particles of a SUSPENSION are so coarse that they settle 
to the bottom of the container when stirring stops.  The solid in 
COLLOID is ground down to a size of about 1/1000 millimeter or less, so that 
thermal motion, electrostatic repulsion and other forces usually keep the 
particles aloft.  Yet even in a colloid, there are still identifiable chunks 
of  matter present, though "visible" only under the electron microscope or 
indirectly by light scattering.

In the case of SOLUTIONS, on the other hand, the "fineness of division" 
goes down right to the atomic (viz. molecular) scale: i.e. millionths of a 
millimeter.  Solutions of A in B can only be made if A is soluble in B: 
i.e. if the molecules of B have an "affinity" to those of A which surpasses 
the affinity of the A and B molecules among themselves.  The solubility, 
however, is also influenced by the increase of "disorder" which happens 
upon dissolution.  Disorder, as we all know, tends to arise spontaneously. 
So if you put sugar in your coffee, at the very first moment you get a
suspension, which spontaneously transforms into a solution, because sugar 
is soluble in water. If you do the same with sand, you can wait forever
because sand (silicon dioxide) is practically insoluble in water.

Best Regards
Werner Sieber

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