Which phase in paper chromatography is stationary?

In paper chromatography, the stationary phase is crucial for separating the components of a mixture. In this setup, the paper itself acts as the stationary phase. This means that while the solvent moves through the paper, transporting the different compounds within the mixture, the paper remains fixed in place.

As the solvent travels up the paper, it carries the compounds with it at varying rates depending on their solubility and affinity for the stationary phase (the paper). Those that are more attracted to the paper will move slower than those that are more soluble in the solvent. This difference in movement allows for the separation of the substances, which can then be identified based on how far they have traveled along the paper compared to the solvent front.

The other components in the question play different roles: the solvent serves as the mobile phase that transports compounds, the compounds themselves move within the solvent, and the atmosphere is irrelevant in this context. Therefore, the stationary phase being the paper is what enables effective separation of compounds in paper chromatography.