Chromatography is used to separate chemical mixtures. If you are separating coloured compounds like ink, dyes or food colouring you can easily see the results by eye. You can also separate non-coloured compounds, like amino acids or sugars, and detect the results afterwards using staining techniques.

As the solvent (in our case water) moves up the paper, it takes the mixture with it. Different components of the mixture will move at different rates depending on their affinity to the solvent i.e. are they hydrophobic (water-hating) or hydrophilic (water-loving). Changing the solvent (e.g. using methylated spirits) gives significantly different results due to the chemical properties of the dyes.. try this with the permanent ink markers instead!

The water moves up the paper by capillary action. Water is sticky, so we are dealing with the forces of adhesion (water sticks to the paper) and cohesion (water sticks to itself).

Note

One common incorrect explanation is that the ink molecules move different distances depending on their size (smaller ones go further, larger ones not as far). While this isn’t true for this experiment, it is part of the reason behind the separation of DNA and proteins by gel electrophoresis (the other part being their charge).

Variables to try

• Try coffee filter paper vs. paper towelling
• Try permanent vs water-based pens
• Do biro pens work?
• Try different solvents such as water, methylated spirits, isopropyl alcohol or turpentine. Which inks respond to which solvent?

More about variable testing

Extension idea

Use this science experiment as part of a mock forensic investigation! Create some ink evidence that can be tested using chromatography… can investigators determine who wrote the ransom note