A student extracts pigments from a leaf and separates them by thin-layer chromatography. The stationary phase is aluminium oxide coated on a glass plate. The student draws a pencil baseline near the bottom of the plate and applies small spots of the extract along it. The plate is placed in a tank containing a non-polar solvent, with the solvent level below the baseline. The solvent is allowed to rise up the plate until it reaches a marked solvent front near the top. The plate is then removed and dried.
Two coloured spots are seen on the dried plate. Spot X has moved halfway between the baseline and the solvent front. Spot Y has moved only one quarter of the distance between the baseline and the solvent front.
Describe the meaning of the terms baseline, solvent front and Rf value, and explain why spot X has a higher Rf value than spot Y.