What is the primary principle behind paper chromatography?

The primary principle behind paper chromatography is based on different solubility levels. This separation technique relies on the varying ability of substances (usually pigments or dyes) to dissolve in a particular solvent. When a mixture is placed on a strip of paper and solvent moves up the paper via capillary action, different components of the mixture travel at different rates due to their differing solubility in the solvent. The more soluble substances will move further up the paper compared to those that are less soluble. This results in the separation of the components as they reach different positions on the paper.

The other options involve physical properties that are not the basis for separation in paper chromatography. Boiling points, densities, and freezing points can influence substance behavior under different conditions but do not significantly contribute to the separation process in the context of paper chromatography as solubility does.