What is the main principle behind chromatography?

The main principle behind chromatography is the separation of a mixture using a medium with varying rates of movement. Chromatography exploits the differences in the movement or partitioning of components within a mixture as they travel through a stationary phase (like a solid or a liquid) and a mobile phase (usually a liquid or gas). This technique allows for the components to be sorted based on their interaction with the stationary phase and their solubility in the mobile phase, leading to the effective separation of the substances.

Different substances in the mixture will move at different rates; some will adhere more strongly to the stationary phase and thus move more slowly, while others may have a stronger affinity for the mobile phase and move faster. This differential movement allows for the individual components of the mixture to be isolated and analyzed.

The other options provided do not accurately represent the fundamental concept of chromatography. The first option refers to physical separation methods that don't encompass the interaction of phases seen in chromatography. The third option relates to a different type of investigation concerning solution properties, while the fourth option is focused on a physical phenomenon unrelated to chromatography, thus making the second option the clearly correct choice.