This is because the nonpolar pigments would just simply dissolve in the nonpolar solvent instead of traveling up the TLC plate.
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The pigment spot on chromatography paper should not be submerged in solvent because it will result in spreading and mixing of the pigments, making it difficult to distinguish them. Instead, the solvent should be allowed to slowly travel up the paper by capillary action, separating the pigments based on their solubility and mobility.
The two types of paper chromatography are ascending chromatography, where the solvent moves up the paper, and descending chromatography, where the solvent moves down the paper.
Submerging the chromatography paper above the pencil line would cause the solvent to rise above the line, potentially leading to the ink or pigment above the line to dissolve and be carried upwards with the solvent. This can result in the separation of different components within the ink or pigment.
Solvent is used in paper chromatography to carry the sample mixture along the paper and separate its components based on their affinity for the solvent and the paper. As the solvent moves through the paper, it dissolves the components of the sample and allows them to separate based on their solubility and interactions with the paper.
The Rf value indicates the ratio of the distance a pigment travels compared to the distance the solvent travels on a chromatography plate. The higher the Rf value, the more soluble the pigment is in the solvent used for the chromatography.
The retention factor (Rf value) in paper chromatography is calculated as the distance the pigment traveled divided by the distance the solvent front traveled. The Rf value is unique for each pigment and helps identify and compare different pigments based on their mobility during chromatography. Pigments with higher Rf values move further up the paper, showing greater solubility, while pigments with lower Rf values stay closer to the solvent front.