posted by Judy .
You and another student were each given an unknown compound. Both samples contained colorless material. You each used the same brand of commercially prepared TLC plate and developed the plates using the same solvent. Each of you obtained a single spot of Rf=0.75. Were the two samples neccessarily the same substances? How could you prove unambiguously that they were identical using TLC?
You can run TLC plates with more solvents and compare the Rfs. An alternative that works well is to spot the two compounds together on the corner of a square TLC plate. Run the TLC plate in one solvent (e.g. the one in the question) so that the spot is about 2/3 of the way along the plate. Now turn the plate thro' 90 degrees and re-run the plate with a different solvent in the new direction. If you still have one spot then they are likely to be the same material.
When running separate plates you could also use spray reagents and UV to visualise the spots. Different materials often have slightly different colours/reactions to the visualisation methods.