Colloids and True Solutions

[B18]

Hello in class we recently were asked to find a test that distinguishes between a colloidal gel and a true solution. The gel we made in lab was sterno. I thought coagulation or Tyndall effect but not confident in either. Any help is appreciated greatly.

 

[Decimus]

The Tyndall effect is probably what your teacher is looking for. It's simple to administer (just shine a light) and works for all classes of colloids (sol, foam, aerosol, etc.) Still, it could be hard to distinguish solid sols from solid solutions if both are opaque.

Coagulation usually only works when liquids are the continuous medium (the medium where the colloidal particles are dispersed in). Furthermore, you have to pick the correct coagulant. A lack of coagulation does not necessarily indicate the unknown is a solution, as you could be using the wrong coagulant.

The last is to use a semipermeable membrane (i.e. dialysis). This is the historical definition as used by Thomas Graham -- he defined colloids to be substances that cannot pass through a semipermeable membrane and crystalloids (solutions) for those which can. Dialysis can be done when the continuous medium is a liquid or gas.