Monday, June 04, 2012

CH Is Something Blue

The answer to yesterday's puzzler is CH, the free radical, whose fluorescent emission makes flames look blue.  The spectrum below is from a butane torch flame.  The CH dominates in the blue (~400 - 500 nm) with some contribution from the blue-green Swan bands of C2.  More about that below.  Contrary to the otherwise pretty great video that won the flame prize, the blue color at the bottom of the flame is not atomic emission.

 As is the case with atmospheric chemistry, the observed reactions are dominated by reactions of free radicals as the Bunny showed in the Rabett Run post on methane oxidation in the atmosphere. Flames have much higher concentrations of free radicals, but also, because the temperature is higher, reactions of stable molecules which require a significant amount of energy to occur, are allowed.  This results in a much richer (and more complex) chemistry.   For anybunny who is worrying, the CN forms because of a small amount of reaction of nitrogen.  The hotter the flame, the more CN will form, but then it will quickly react away to form NO.

There is a lot to learn from flame chemistry and it has real consequences for optimizing all sorts of engines and industrial processes.  The US DOE has established several laboratories for this, among which the best, IEHO, is the Combustion Research Facility at Sandia, CA.  Eli is greatly in favor of efficiency.

Rabett Run will be returning to this theme several times this week to discuss other flaming issues.  (NONONO not that kind of flame, candle flames)


Hank Roberts said...

Kids, try this at home, you can find directions online for building your own spectroscope out of a cardboard tube and DVD.

For adults, there's a kit:

Mark said...

425 nm is closer to purple than blue. Thus, the more intense emission from CH would suggest that the flame would appear purplish. However, the eye's spectral response is much less sensitive to purple than to blue (, so despite the greater intensity of purple-blue light from CH, our eyes (and brain) respond more strongly to the blue light from the Swan bands.

Aaron said...

The video asks me to think of something, so I thought of the sun. What do we tell an 11 year old that the sun is made out of?

Thus, I assert that the video contains more than "a" mistake.

When I was a kid they told me all kinds of simplistic garbage. Then I had to unlearn it all and learn the right stuff. It would have been better to just learn the right stuff in the first place.

EliRabett said...

It is really hard simplifying things so that young people can understand them without screwing up something that come back to bite years later

Horatio Algeranon said...

One thing you have to bear in mind here is that the winner of the flame challenge was chosen by kids and while the "mistake" might seem "huge" (Eli's word) to a college professor of chemistry who studies this stuff for a living, it's debatable whether it is all that important in this case.

We're talking about 11 year olds here and at that age, concepts are far more important than details (eg, about whether the blue light comes from excited atoms or from excited 'free radicals").

And yes, as anyone who has ever tried to simplify science so that kids will not only understand it but be interested (and even entertained) knows, it ain't easy.

In many regards, one has to actually understand it better to explain it to kids than to explain it to adults.

On the whole, Ames did a great job, not only from a science standpoint, from a teaching standpoint.

He obviously is well aware of the fact that different kids learn in different ways -- eg, some through pictures and some through words.

Again, unless one has actually tried to teach kids, it may not be obvious what a good job Ames did.

Anonymous said...


This sun is made of ionized gas---positively and negatively charged atoms. It is a plasma. Plasmas are made of atoms--charged atoms. So you can tell an 11 year old that the sun is made of atoms. I hope that helps!