What is considered the mobile phase in paper chromatography?

In paper chromatography, the mobile phase is defined as the solvent that moves through the paper. This is crucial because the mobile phase carries the substances being analyzed along the length of the paper as it moves. As the solvent rises up the paper, different compounds travel at different rates due to their varying solubilities and affinities for the stationary phase (the paper itself). Thus, the choice indicating that the solvent is the mobile phase accurately reflects the fundamental principles of the technique, where separation of substances occurs based on their interaction with both the mobile and stationary phases.

The other options do not represent the mobile phase correctly. The paper serves as the stationary phase, providing a surface for the compounds to adhere to as the solvent moves. The compounds being analyzed are the substances that are separated during the process, while the container holding the chromatography setup is simply a physical structure that houses the entire apparatus and does not influence the movement of the solvent or the separation of the compounds.