Dry-erase markers — and the teachable moment

Dry-erase markers — and the teachable moment

I just had the most wonderful Grade 11 Chemistry lesson that really took off in the final few minutes. 

The topic was an introduction to solubility — “like dissolves like”.

I began with a side-by-side demonstration: one separatory funnel contained hexane (clear) atop KMnO₄(aq) (purple); the other had I₂ dissolved in hexane (purple) atop pure water — see Fig. 1. 

With a bit of showmanship, it appeared as if the purple layer moved from top to bottom (or vice-versa).¹ 

This led to a fruitful discussion about density, solubility and a bunch of things I can’t remember. 

We talked about “Goo-Gone”, a non-polar solvent,² and how it can be used to remove the gunk that remains when a price sticker is removed. On a whim, I took a Sharpie³ and wrote on the white board. There were gasps; they knew the board eraser wouldn’t rub that ink off. 

It didn’t. 

Neither did a wet paper towel.

Fig. 1 Left: Hexane layer atop an aqueous layer with KMnO₄. Right: Iodine dissolved in hexane layer atop an aqueous layer.

So the question was — is the Sharpie-ink polar or non-polar? 

Non-polar, it looks like, Sir.

It didn’t take long for someone to suggest hexane as a solvent to remove the Sharpie-ink, which it did beautifully. Another student added that if one writes overtop of Sharpie-ink with a dry-erase marker it will be erasable as usual, which totally worked. 
 
Next question: is the dry-erase marker polar or non-polar? 

Evidence suggested that is non-polar, but how could we verify? 

I stirred a dry erase marker-tip in a beaker of water; no ink dissolved. But when another marker-tip was stirred in hexane, the dye dissolved — evidence that the ink from a dry-erase marker is non-polar. The ink can be friction-removed; wiping with a wet paper towel also works, but not because of the water.  

This was all very exciting — a beautiful, practical, engaging kick-off to a larger lesson on intermolecular forces.

References

1. This is not my idea. I’ve lost the reference, but I made a nice handout. Email Jean Hein for handout, jhein@uwaterloo.ca. 

2. Goo-gone is, like, 95% petroleum distillates, which sounds a lot like hexane. www.homedepot.com/catalog/pdfImages/d6/d6a2c387-5297-46a7-9dbb-eba31f3bb9e3.pdf

3. A permanent marker favoured by elementary school teachers and graffiti artists.   

[Editor’s note: With any demonstrations investigate the SDS information and all the potential safety issues. For example, this demo was done in a fume hood because of hexane’s toxicity, volatility and flammability. Potassium permanganate solution and iodine are both corrosive and hazardous. The taps and the stoppers of the separatory funnels are always checked for leaks before a demo like this. One proofreader suggested making this demo possible outside the fume hood by using chemicals such as mineral oil and food colouring. Using another glue solvent, which is not 95% petroleum distillate, could also be considered.]