Pen chromatography science experiment | Fizzics Education

Pen chromatography

Pen chromatography

Follow FizzicsEd 150 Science Experiments:

You will need:

A glass of water

Different coloured felt tip pens

White paper towelling

Scissors

A plastic cup of water, paper towelling, textas an strips of paper towelling on a black desk in front of a green background
1 A strip of white paper on a black table cloth

Cut a thin strip of white paper towelling using your scissors.

2 Drawing a black line using a felt tip marker across the bottom of a white paper toweling strip

With one of the felt tips pens, draw a line across the paper towelling strip roughly 1cm from the bottom of the paper.

3 A hand holding a paper strip so that the end just touches the surface of water in a glass. Water is beginning to rise through the paper towards the black line drawn on the paper

Carefully place the paper strip into the water so that only the edge of the paper touches the surface of the water. Water will rapidly rise up through the paper due to capillary action.

4 Water spreading a black line drawn on paper. The ink is rising upwards as the water spreads up the paper strip

Once the water reaches the ink line, lift the paper strip off the surface of the water. You’ll notice that the ink rises upwards as the water comes up through the ink line. If you look closely, you might see some colours coming out of the line that you didn’t see before!

5 Spread out black ink on whote paper, showing a thin blue line on one edge of the black ink streak

The longer you run the chromatography experiment, the more the colours spread out across the paper.

6 5 strips of paper, each with different coloured inks spread out across them showing lines of colour in each

Try different felt tip pens for your chromatography investigation! Purple, orange and black inks tend to have hidden colours within them … which other colour inks also have hidden colours?

What is happening?

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?

Video of this experiment during a Facebook Live presentation

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