Funny felt pens

You can buy felt pens which show one colour when used normally, but give a second colour when you go over them with a “magic pen”. 

This experiment should take 70 minutes.

Equipment

• Eye protection
• Stoppered boiling tube into which filter paper strip can be inserted
• Beakers, 100 cm³
• Petri dishes
• Glass droppers
• Scissors
• Paper clips
• Pencil
• Ruler (measuring to mm)
• 1 packet of ‘Funny’ felt pens – many branded and non-branded ones are available – any that change colour when you go over them with the magic pen should work   
• Whatman grade 1 chromatography paper 100 m roll, 30 mm wide (Alternatively ordinary filter paper may be used.)

Solvents

• Water
• ethanol
• Propanone (acetone)
• Salt solution [a 0.1 wt% salt (NaCl) solution has been used as a solvent to separate food dyes by paper chromatography]. 

Health, safety and technical notes 

• Read our standard health and safety guidance here.
• Wear eye protection.
• Some pen inks can stain clothes, wear lab coats, or aprons if desired.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• No naked flames.
• Propanone (acetone) is highly flammable and an eye/respiratory irritant. See CLEAPSS Hazcard HC085a.
• Ethanol is highly FLAMMABLE (if methylated spirits are used, they are also harmful if ingested and cause damage to organs). See CLEAPSS Hazcard HC040a.

Possible approaches

Paper chromatography of the coloured pens. Colour changes are due to pH changes of dyes/indicators (Vogel’s Textbook of “Quantitative Inorganic Analysis” Fourth edition, p240-241 gives a good list of indicators).

The “Magic” pen contains alkali. One approach might be to separate colours from a “funny” felt pen using paper chromatography, then dry the chromatogram (a hairdryer is handy) and dab the ‘magic’ pen on the separated colours to see which of the colours it affects.

Perhaps put out acid and alkali as resource chemicals as a hint to lead students into thinking that the problem involves acid/base chemistry. 

Possible extension

Students could try to identify the names of indicators in each pen – question involving pH charts.

Are ‘ordinary’ felt pens susceptible to the magic pen?