What is the name for the highest point the mobile phase reaches on the filter paper?

The highest point the mobile phase reaches on the filter paper in chromatography is called the "front" or "solvent front."

In chromatography, particularly in techniques like paper chromatography, a solution (the mobile phase) moves along the stationary phase (the filter paper) due to capillary action. As the solvent travels, it carries along various components of the mixture at different rates, leading to the separation of the components. The solvent front is crucial for determining the retention factor (Rf) values for each component, which is essential in analyzing and identifying substances within the mixture. The distance traveled by the solvent front is compared to the distance traveled by the solute spots to characterize the separation process effectively.