What is defined as the phase that moves in a definite direction in chromatography?

In chromatography, the mobile phase is the solvent or gas that carries the samples through the column or along the stationary phase. It plays a crucial role in the separation process by moving in a definite direction, which allows the various components of the mixture to interact differently with the stationary phase. The difference in how each component interacts with both the mobile and stationary phases leads to distinct migration rates, resulting in separation.

The stationary phase, on the other hand, remains fixed in place, providing a surface for interactions with the sample components. The equilibrium phase is not a standard term used in chromatography to describe the motion in the separation process, and reactive phase generally does not apply to the description of how phases behave in chromatography. As a result, the mobile phase is the correct answer, as it is defined by its movement and function in the chromatographic separation process.