Fond of bonds

Les Trois Danseuses, Picasso, 1925

O body swayed to music, O brightening glance,
How can we know the dancer from the dance?

W.B. Yeats, from "Among School Children"

***

I teach biology, or try to, anyway. What I mostly teach, I think, is a way to look at the world, a way labeled "science."

Through a series of events mostly inexplicable to me, as much related to the state biology exit exam as to the retrograde motion of Mercury, my sophomores have had essentially no chemistry before coming to my class. None...

I get to expose them to chemistry. I love chemistry. To grasp chemistry, you need to grasp bonds.

Trouble is, there is nothing to grasp--in a sense, they do no exist.

***

I had a lot of trouble with the concept of potential energy when I was in high school. A teacher would lift an object, the pontificate about how this object now had "potential" energy.

Where, I wondered (and eventually learned not to ask), was the energy?

It's like trying to "see" the energy in the water at the top of a waterfall. The water itself is exactly the same at the top as it is at the bottom. I could see that a waterfall could do work, I am not stupid, but where was it before the molecules cascaded over the dam?

I was told the energy was in the object until I finally got my first real science teacher, Ms. Lehman, a wonderful woman with a wonderfully twisted view of reality (any honest view will be twisted) and the patience of Job, who explained to me that the energy was in the position. Over and over again until I got it.

Once I internalized this, I shot through chemistry and never looked back.

It is really a simple, simple idea.

***

The waterfall analogy suffers from a major flaw--rearranged atoms do form new substances. Still, you end up with exactly the same atoms that started the reaction, just scrambled up differently.)

Every time.

If you hope to get kids (or other forms of humans) to understand chemistry, they must also internalize the Law of Conservation of Mass and Energy. This can take months, even years, especially if our children demand evidence. (Alas, we beat the snot out of the curious very early on in public education...)

We don't have that kind of time--so we (students, teachers, districts, states, with Arne's approval) fake it.

I suspect that's a major reason so many despise stoichiometry, a shame--stoichiometry is the Mikhail Baryshnikov (or Michael Jackson) of science, an intricate choreographer of seemingly impossible moves as energy flows through matter.

The moves of Michael Jackson do not "exist"--they represent the relative positions of pieces of Michael Jackson.

***

Many of us "teach" the photosynthesis/respiration equation this way:

C₆H₁₂O₆ + 6O₂ ⇒ 6CO₂ + 6H₂O + energy

Kids see "energy" the same way they see everything else there, if they see it at all. Energy is hanging out there just like the other "stuff"--heck, they've been told since kindergarten that plants convert sunlight into food.

Plant do not do any such thing--they simply rearrange the stuff around into more complex, less stable forms.

We tell the more sophisticated students that plants put energy is "in" the bonds, and they nod sagely, writing  down like ancient Irish monks bent over their vellum, recopying wisdom passed down through the ages.

I'm not here to tell you the Irish copied bunk--I do not need my Granny's leathery yellowed hand breaking through the earth, grabbing my ankle. I will tell you, though, that our student do.

***

Here's what I do.

Every time I mention a reaction that requires a net input of enery to build a bigger, less stable molecule, I stack a lab stool on top of a desk. It takes exertion. When I'm done, I have the same stuff--a desk and a stool, but I have a larger, less stable object.

If I want to break it down, I still need to put in a little bit of energy to nudge the structure. This is not a trivial matter. If a few atoms are clustered together in a non-random position, there's something about that position that allows them to "stick" together. Every reaction requires breaking up that something.

We call this activation energy.

I love watching the stool cascade off the desk, loudly bouncing along the floor until it comes to rest in a more stable position. The kinetic and sound energy are obvious, the bump up in temperature where the stool collided with the floor a little less so. ( I've been know to touch the floor at the spot of impact and pretend it is hot.)

Energy released.

The stool lying sideways on the floor is pretty darn stable, and its position pretty darn strong. This may be counter-intuitive--students confound strength of bonds with the energy released as substances break down from less stable to more stable arrangements.

This is, of course, a bit of a simplification, but unlike the concept of bond as some thing betweeen atoms, it allows for growth of a more accurate model. It also makes visible the idea of potential energy as a consequence of position, of the dance, as opposed to the stuff itself, or the dancer.

The Michael Jackson photo is from Spilled Mind, without attribution.

You cannot appreciate MJ's genius from a still.

Elementary science: playing with fire

Fire is obvious, so it seems. Pretty much every child recognizes the flame of butane lighter is the same as the flames on the stove or on a lit candle.

A child sees that a fire makes solids things smaller. The grown-ups tell children that fire consumes, that the logs burned up, that fire reduces things to ash.

And pretty much every adult who believes this still lives in the world of alchemy, hoping to turn lead into gold.

My September sophomores know what fire is, no surprise, since September sophomores know everything there is to know about anything.

Before I ever say the words respiration or calorie, I ask them about fire—a few look confused (a good sign in science class), but most give me a knowing smile—they know what it is, they “just can’t put it into words” and when they do, they describe the properties of fire. Not a bad start.

I ask them what you need for a fire, and they know that—fuel, oxygen, something to light it—somewhere in elementary school they learned about the fire triangle.

I then pretend to take out a box full of pure oxygen, and ask them what would happen if I lit a match in it.

***

Most of my sophomores know the photosynthesis/respiration equation before they get to my class:

C₆H₁₂O₆ + 6O₂ => 6CO₂ + 6H₂O with energy released

Sugar + oxygen combined releases carbon dioxide and water

CO₂ + H₂O => C₆H₁₂O₆ + O₂ with energy captured

The kids love writing down equations, it gets them feeling all sciency, and now the stupid teacher isn’t asking stupid questions about stupid fire expecting answers that “can’t be put into words.”

The inevitable “Do we have to know this?” comes from the back corner of the classroom—always the same back corner—but I pretend I don’t hear.

I hold up my propane torch—even the back corner crowd notices now. I promise them I will light it in a minute, but they have to answer a simple couple of questions first. What do I need to make it work. (“Well, duh…”), and what is H2O (“Well, duh…” with an advanced eye roll).

I write the equation for the combustion of propane on the board—it’s similarity to the respiration/photosynthesis equations is glaringly obvious, but not a point I care to make at the moment.

C₃H₈ + 5O₂ => 3CO₂ + 4H₂O

I ask what comes out of the propane torch after the propane as the propane is burned. I consistently get two answers—fire and carbon dioxide. I never get water. I’ve asked hundreds of kids the question, with the equation sitting up on the board, and it’s like H2O is some mysterious stuff stuck to the equation just to make it balanced. The stuff is pretty mysterious when you get down to it.

After our list of stuff that comes out of the torch is made—usually CO2, heat, light, flame, and occasionally propane—I light the torch.


I pass the torch over a cool piece of glass—it could be a large beaker—then pass it over the cool stem of the faucet. The students see the flash of water vapor on the glass. They know it looks like "fog” — but no one wants to say it. It makes no sense. Water from fire? It must be a trick.

To be fair, it pretty much gobsmacks me, too, each time I do this.

And of course, water does not come from fire—it comes from the hydrogen in the propane and the oxygen in the air. Turns out we’re all closet alchemists. We cannot accept the obvious.

***

Chemistry hit puberty  when Antoine LaVoisier realized that fire consumes nothing—it only transforms. If you figure out the amount of stuff with and compare it to the stuff you end up with, it has the same mass.

Exactly the same mass.

All the heat and light and noise that escaped from the dancing flame took nothing away. Energy has no mass, no inertia, no stuff to it. It's not nothing, but it's not mass, either.

So what do we teach a young child about fire? Let them observe a candle, let them see the water rise from the flame, let them cover it with a glass and see the flame die, let them wonder.

Matchstick photo by Sebastian Ritter via Wikipediaa under CC.