A student is separating the photosynthetic pigments from a spinach leaf extract using thin-layer chromatography (TLC). After spotting the extract on the silica gel plate, they place it inside a beaker containing a mobile phase of petroleum ether and acetone. The student ensures that the beaker is sealed with a watch glass on top.
Explain why placing a lid on the beaker is necessary for the chromatography to run successfully.
