Typically the solvent used for the chromatography is rather volatile .
Thus, an open beaker will cause the solvent mixture to more quickly diverge from the intended mixture for the chromatography,
which will detract from chromatography and likely harm the usefulness of the separation.
The reason for covering the beaker is to make sure that the atmosphere in the beaker is saturated with solvent vapour. To help this, the beaker is often lined with some filter paper soaked in solvent. Saturating the atmosphere in the beaker with vapour stops the solvent from evaporating as it rises up the plate. As the solvent slowly travels up the plate, the different components of the dye mixture travel at different rates and the mixture is separated into different coloured spots
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