Chromatography PAG

• Why must the origin line be drawn in pencil, not pen?

  Ink from the pen would dissolve in the solvent and run (leads to inaccurate results)
• Why should you draw a line where the solvent reached immediately?

  Solvent evaporates rapidly so the end position is not visible shortly after the experiment.
• Why should you measure the rf value from the middle of the pigment mark?

  The pigment mark is spread out, so measuring from the middle position standardises the measurements you are taking to allow for comparisons
• Why should you make sure your chromatography paper is vertical and straight?

  So the pigments move straight up the paper to avoid them running off the side of the paper and/or being washed off.