[magnetic111]

Im not sure when I should be using sephadex column chromatography or silica gel column chromatography to separate compounds?

[Babcock_Hall]

Silica gel chromatography separates molecules based on their relative attraction to silica, which is generally greater for more polar molecules than for less polar molecules.  Sephadex is often used to separate molecules (especially biomolecules) on the basis of size.

[Babcock_Hall]

What are you trying to separate?  It would be easier to figure out the best strategy with a specific case.

[magnetic111]

We weren't given a specific sample to separate.  It is a question put to us by our lecturer...

[Arkcon]

Then can you use the information already given to you by Babcock_Hall: to create a sample for each type of column?  Pick two compounds that are separate-able based on the chemical interactions mentioned.  This would be a great way to prepare for an upcoming exam, you can use your custom made examples in a definition.