CENtral Science

How Would You Explain pH to First-Graders?

So, this was the dilemma I encountered this week at the day job.

In my position at the North Carolina Museum of Natural Sciences in Raleigh, we give public “Meet the Scientist” talks twice daily in our iconic multimedia space called the SECU Daily Planet in the new Nature Research Center wing of the Museum. The Daily Planet Theater seats about 50 folks on the main floor but is open on part of the 2nd and 3rd floors for visitors to peer into events there.

The SECU Daily Planet, Nature Research Center, North Carolina Museum of Natural Sciences, Raleigh, NC. Photo: Robert Carpenter, NCMNS Flickr

Led by my science communications partner, Brian Malow (@sciencecomedian), these free public sessions are designed as short (10-15 min) talks by museum scientists or as live interviews conducted by Brian. In both cases, the goal is for our visitors to, well, meet the scientists. Specifically, the public get to meet an actual scientist by asking them questions, learning about their motivations for entering their field, and what their days are like. I find it quite powerful for visiting schoolkids to talk with scientists about the things in their childhoods that led them to be astronomers, paleontologists, geneticists, or zoologists.

While I’m the science communications director, I’m still considered enough of a scientist to do these Meet the Scientist sessions. I’m using my opportunities to discuss chemistry of the natural world and where folks encounter cool chemistry in their daily lives.

On Wednesday, I was just about to give a talk on the maytansine analog that serves as the cytotoxic payload for the new Roche/Genentech/ImmunoGen antibody-drug conjugate, Kadcyla. Brian came rushing in right before to warn me that we had a slew of visiting first-graders and that I might want to give one of my more fun, age-appropriate talks.

So, we decided to explain and demonstrate the concept of thermochromism, the property of some compounds to change color in response to temperature change. I came up with this talk based heavily on this excellent post by anmanam at Chemistry Blog which he based on this J Chem Ed paper by Mary Anne White and Monique LeBlanc from the Department of Chemistry at Dalhousie University. The Museum Gift Shop fortunately carries three colors of thermochromic plastic cups which become darker with cold.

David Kroll and Brian Malow with first-grade volunteers demonstrating thermochromism. SECU Daily Planet Theater, Nature Research Center, NC Museum of Natural Sciencesl 6 March 2013. Photo: Screenshot of MP4 taken by Eban Crawford, Museum Digital Media Specialist.

Brian and I invited four students to come up to demonstrate the concept, with one unfortunate students having the control cup without thermochromic dye. As you might guess, explaining the concept was challenging. After all, the White and LeBlanc paper suggests using this as a first-year undergrad lesson.

These are the money paragraphs from azmanam:

The prototypical thermochromic ink is crystal violet lactone.  When the pH is high, the lactone interrupts the conjugation that would otherwise extend through all three aromatic rings.  When the pH is low, the lactone becomes protonated, and the lactone opens to the carboxylic acid, leaving a tertiary, benzylic carbocation behind.  This carbocation allows the conjugation to extend throughout all three aromatic rings.  With the conjugation extended, λmax increases into the visible region, and the leuco dye appears, well, violet colored.

For inks, the equilibrium is controlled in a very clever way.  We can’t constantly be dousing our beer bottles with acid or base depending on which color we want.  For thermochromic inks, the manufacturers take the lecuo dye, some weak acid, and a high molecular weight solvent and encapsulate the components into a particle usually <50 μm in diameter.  The leuco dye chosen will depend on what color is desired. Weak acids typically chosen include bisphenol A (yes, that BPA), octyl or methyl p-hydroxybenzoate, 1,2,3-triazoles, or 4-hydroxycoumarin derivatives.  The solvent is typically an alcohol (laurel or cetyl alcohol), an ester (butyl stearate), a ketone, or an ether.  The melting point of the solvent is important.  The melting point of this solvent is the temperature at which color change will take place.

All I tried to tell the first-graders was that temperature changes the pH, or acidity, inside of the plastic. And we kept repeating “thermochromics” so that they associated the temperature change with the color change. (Although azmanam reminds us that the correct term is halochromism since the color change is really due to changes in protonation of the leuco dye.

But, you know, it’s tough to describe pH.

Most people, even kids, have heard of acids. But few are conversant in the concept of bases, much less acid/base equilibria. I tried to use the comparison of orange juice, water, and toothpaste to demonstrate the broad range of pH. I didn’t even bother trying to tackle the idea that parts of the cup were microscopically melted at room temperature and solid when cold.

Next time, I may not bother even discussing pH. But if I were to, how would you explain the concept of pH, or acids and bases, to first-graders? (or to adults, for that matter!)

By the way, the North Carolina Museum of Natural Sciences was recently named the most-visited museum or historical site in the state, with 1.22 million visitors in 2012. This was the first time in nine years that the museum came out on top of the previous top attraction, Biltmore Estate in Asheville.