Last week, Katie Burke shared her recipe for vibrant spring Violet Jelly at The UnderStory. [1] Apparently, steeping violets produces a blue-green liquid that Katie changed to a more appetizing pink-purple using lemon juice. Katie explains that the color of the anthocyanins (which are pigment molecules) in violets is influenced by pH.

I began to wonder a few things.

1. How does the pH change the color?
2. What exactly is pectin?
3. What was the secret of that “oscillating clock” reaction we used to do at the Chemistry Magic Show back in undergrad?

The first question led me through memories of pH indicators to an excellent explanation [2] that I will attempt to summarize as simply as possible here.

1. The color of light is related to the amount of energy in a photon of that light.
2. Molecules can only exist in certain energy states.
3. Therefore, molecules can only absorb light that has photons of a certain size. [3]
4. Different kinds of molecules absorb different-sized photons.
5. If electrons in a molecule are confined, the energy gaps of the molecule spread out; this translates into an ability to absorb larger photons of light.
6. Conversely, if the electrons are able to spread out in a molecule, the energy gaps are smaller and the molecule absorbs smaller photons of light.
7. Larger energy light absorbed = bluer; smaller energy light absorbed = redder.

If this is all totally confusing, imagine an elevator. The different floors of the building are the energy levels of a molecule. The elevator is programmed so it can only stop at a floor, not between them. The elevator gets packets of energy that allow it to climb up. One-sized packet carries it from floor 1 to 2. Another might carry it from 2 to 3 or even from 1 to 3. The size of the energy packets needed matches the spacing between the floors and would change if the building had different spacings… i.e., if it were a different molecule.

The general equation that illustrates pH indicators is this:

HIn ↔ H+ + In-

On the left side is the molecule (In) bonded to H, and on the right side is the molecule with H removed. On the left side, the molecule’s electrons are confined into a bond with that stupid killjoy hydrogen, but on the right side they are happy and free. So what happened with Katie is…

1. Happy free electrons = absorb reddish light.
2. Add acid; reaction shifts to left side.
3. Confined electrons = absorb bluish light.

But wait a minute. Katie’s jelly turned more RED when she added lemon juice. What’s up with that? Because the molecules absorbed bluer light, red was what was leftover to shine out from the depths of her jelly.

To see the actual molecules at work and get more references, visit [2] and scroll down. I’ll tackle my other questions in another post.

[1] “April Showers Bring Jelly Made Out of Flowers,” The UnderStory, by Katie Burke, 2014. http://www.the-understory.com/2014/04/22/april-showers-bring-jelly-made-out-of-flowers/

[2] “Water to Wine. The molecular basis of indicator color changes,” General Chemistry Online. http://antoine.frostburg.edu/chem/senese/101/features/water2wine.shtml

[3] UPDATE: My dad pointed out that photons do not have physical “size.” Using the correct terminology was never my strong point! What I mean is the amount of energy in the photon, and I sloppily use “big” or “large” for “more energy” and “small” for “less energy.” I write it the way I picture it, I guess because I’m trying to help others picture it, but I don’t mean to be inaccurate.