Separation of solutions and mixtures chromatography

A chemist is separating a mixture of two substances, X and Y, using column chromatography. The chemist loads the mixture onto a column filled with silica gel (a polar material) and then flows a nonpolar solvent through the column. Partway through running the column (at $t=t^{\prime }$‍), the chemist observes the separation of X and Y shown below.