A student performs a TLC experiment and at the end of the experiment, sees no spots

In a TLC experiment, if no spots are seen then it might be due to the following reasons:

It might be that the concentration of the sample might be too low. This problem can be overcome by spotting the sample multiple times.There might be a problem with the development method and we shall look for alternative methods.It might also happen that there might be no compound in your sample due to errors in the experiment performed. You should have a re-check of your experiment.The level of the solvent system might be too high. Re-conduct the experiment with right amount of solvent.

covalent compounds are less soluble in water. The tendency of compounds to dissociate or ionize tells about the way in which bonds hold the compound itself together. Covalent compounds aren't usually very soluble in water but the ionic compounds are.