Why are the plates in column chromatography called 'theoretical plates'?

Answer 1

The number of theoretical plates in a chromatography column is a measure of how "long" the column is - how well it separates. A "short" column will only separate large or heavy molecules, and the medium and light stuff is still mixed together in the last band. A "long" column will separate the little stuff better because there are more theorectical plates.

Picture a stack of sieves with smaller and smaller holes as the column gets "longer" and you've got the idea.

This "length" has virtually nothing to do with the physical length of the separating column. It is a function of the packing materials and solvents used during a separation.

Answer 2

The plates in column chromatography are called theoretical plates because they conceptually represent an idealized stage in the separation process where solute molecules are distributed between the stationary and mobile phases repeatedly. Each theoretical plate represents a hypothetical stage of equilibration where solute molecules equilibrate between the two phases.