Candle, Candle, Burning Bright

<chirp, chirp> “Moire here.”

“Hi, Sy, it’s Susan Kim. I did a little research after our chat. The whale oil story isn’t quite what we’re told.”

“Funny, I’ve been reading up on whales, too. So what’s your chemical discovery?”

“What do we get from a fire, Sy?”

“Light, heat and leftovers.”

“Mm-hm, and back in 18th Century America, there was plenty of wood and coal for heat. Light was the problem. I can’t imagine young Abe Lincoln reading by the flickering light of his fireplace — he must have had excellent eyesight. If you wanted a mostly steady light you burned some kind of fat, either wax candles or oil lamps.”

“Wait, aren’t fat and wax and oil three different things?”

“Not to a chemist. Fat’s the broadest category, covers molecules and mixtures with chains of ‑CH2‑ groups that don’t dissolve in water. Maybe the chains include a few oxygen atoms but the molecules are basically hydrocarbons. Way before we knew about molecules, though, we started classifying fats by whether or not the material is solid at room temperature. Waxes are solid, oils are liquid. You’re thinking about waxy‑looking coconut oil, aren’t you?”

“Well….”

“Coconuts grow where rooms are warm so we call it an oil, OK? I think it’s fun that you can look at a molecular structure and kind of predict whether the stuff will be waxy or oily.”

“How do you do that?”

“Mmm… It helps to know that a long chain of ‑CH2‑ groups tends to be straight‑ish but if there’s an ‑O‑ link in the chain the molecule can bend and even rotate there. Also, you get a kink in the chain wherever there’s a –CH=CH– double bond. We call that a point of unsaturation.”

“Ah, there’s a word I recognize, from foodie conversations. Saturated, unsaturated, polyunsaturated — that’s about double bonds?”

“Yup. So what does your physicist intuition make of all that?”

“I’d say the linear saturated molecules ought to pack together better than the bendy unsaturated ones. Better packing means lower entropy, probably one of those solid waxes. The more unsaturation or more ‑O‑ links, the more likely something’s an oil. How’d I do?”

“Spot on, Sy. Now carry it a step further. Think of a –CH2– chain as a long methane. How do suppose the waxes and oils compare for burning?”

“Ooo, now that’s interesting. O2 has much better access to fuel molecules if they’re in the gas phase so a good burn would be a two‑step process — first vaporization and then oxidation. Oils are already liquid so they’d go gaseous more readily than an orderly solid wax of the same molecular weight. Unless there’s something about the –O– links that ties molecules together…”

“Some kinds have hydrogen-bond bridging but most of them don’t.”

“OK. Then hmm… Are the double-bond kinks more vulnerable to oxygen attack?”

“They are, indeed, which is why going rancid is a major issue with the polyunsaturated kinds.”

“Oxidized hydrocarbon fragments can be stinky, huh? Then I’d guess that oil flames tend to be smellier than wax flames. And molecules we smell aren’t getting completely oxidized so the flame would probably be smokier, too. And sootier. Under the same conditions, of course.”

“Uh-huh. Would you be surprised if I told you that flames from waxes tend to be hotter than the ones from oils?”

“From my experience, not surprised. Beeswax candlelight is brighter and whiter than the yellow‑orange light I saw when the frying oil caught fire. Heat glow changes red to orange to yellow to white as the source gets hotter. Why would the waxes burn hotter?”

“I haven’t seen any studies on it. I like to visualize those straight chains as candles burning from the ends and staying alight longer than short oil fragments can, but that’s a guess. Ironic that a hydrogen flame is just a faint blue, even though it’s a lot hotter than any hydrocarbon flame. Carbon’s the key to flamelight. Anyway, the slaughter started when we learned a mature sperm whale’s head holds 500 gallons of waxy spermaceti that burns even brighter than beeswax.”

~~ Rich Olcott

  • Whale image adapted from a photo by Gabriel Barathieu CC BY SA 2.0

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