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IRA FLATOW, host:

Up next, Flora Lichtman, our digital media producer, is here and - with our Video Pick of the Week. Hi, Flora.

FLORA LICHTMAN: Hi, Ira.

FLATOW: What have you got for us today?

LICHTMAN: Well, you know, it's a holiday, so we're going with the double feature.

FLATOW: A double, double feature - that's very nice.

LICHTMAN: First, we have Theo Gray's video of exploding hydrogen bubbles.

FLATOW: Wow. That's cool.

LICHTMAN: It kind of sells itself. I'm not even sure we have to say anything else. I mean, he - he lights hydrogen bubbles on fire.

FLATOW: Right. You can see that at sciencefriday.com on our Web site.

LICHTMAN: Right.

LICHTMAN: That's one.

LICHTMAN: And then, you know, it's Fourth of July, so I thought we should look into fireworks, because it seemed like there was probably a lot of science going on.

FLATOW: Mm-hmm.

LICHTMAN: And so I put a call into the master of science chemistry presentation.

FLATOW: That could be only one person.

LICHTMAN: That's right. It is Bassam Shakhashiri. And he's here with us today. He's a chemistry professor at the University of Wisconsin, Madison.

FLATOW: Hi, Bassam.

Dr. BASSAM SHAKHASHIRI (Chemistry, University of Wisconsin-Madison): Hello, Ira.

LICHTMAN: Hi.

Dr. SHAKHASHIRI: Hi, Flora.

LICHTMAN: So, let's start with - break it down for us. Let's start with the lights that we see in fireworks. Is it fair to say it's just a chemical reaction that's causing these lights?

Dr. SHAKHASHIRI: These are all combustion reactions that release energy in the form of light, in the form of sound, and of course in the form of heat. And so, yes, they're all highly combustible materials. And that's why, you know, listening to the visitors that you had, Ira, on your program, I really enjoyed everything they say. And as you know, I'm a strong advocate of experimentation, hands-on, minds-on. But I like to leave the fireworks to the professionals. I like to watch the fireworks and I like to be sure that everything is done safely.

The colors that are produced are due to different chemicals. There are salts that are put into stars, which are small clay or dough-like lumps or cubes about three to four centimeters in diameter. They are then lifted up to the sky, and when they explode they give off energy in the form of light. So, red is due to strontium. Orange is due to calcium - it's calcium salts, strontium salts. Yellow is ordinary table salt. The green color that we see is from barium chloride. The beautiful blue is copper compounds, copper chloride specifically.

LICHTMAN: And what's actually going on in these compounds when you heat them?

Dr. SHAKHASHIRI: So the heat excites the electrons that are in the metal ions and puts them into a higher, so-called excited state. And when they fall back, then they emit energy, and this energy happens to be in the visible region of the electromagnetic spectrum, and so we get to see it and enjoy it.

LICHTMAN: Okay. So one thing that's always really puzzled me when I go to fireworks displays has been how they create these elaborate shapes. Like, how does - how do you make Mickey Mouse out of these chemicals?

(Soundbite of laughter)

FLATOW: Yeah.

Dr. SHAKHASHIRI: Very, very good question. You know, and this is why I say the professionals are really, really very artistic at doing this. And everything is choreographed so well so that the specific explosions, as they take place, the specific combustion reactions, as they take place, they produce these different shapes as they cascade down. And it's all done very, very artistically by the professionals who pack them so that the combustion process, the explosive process takes place in sequence and creates the visual effect that we all enjoy.

LICHTMAN: So the packing is translated into sort of timing of when these compounds are ignited?

Dr. SHAKHASHIRI: Yes. Exactly. And how - and when they're triggered, you know, depends on the sequence that the professionals who put them together visualized and wants to have us enjoy.

LICHTMAN: So there's a real, sort of, artistry to this?

Dr. SHAKHASHIRI: A great deal of artistry and a great deal of science. It's a beautiful combination of science and art, but it is also dangerous to do it yourself. And as I want to repeat what I said earlier, the importance of sitting back, enjoying the fireworks and if you do some of the experiments that were discussed earlier, do them safely.

I'm a very strong admirer of Theo Gray's work. I love his book, and the pictures that are in his most recent book. And I urge others to also take a look at it. But I also want to remind all of us as we enjoy the fireworks, to remember what we're really celebrating. We're celebrating Independence Day. It is the "Declaration of Independence" that everyone should also read tomorrow as well as enjoy the fireworks.

FLATOW: Well, "Declaration of Independence" is on my reading list every Independence Day.

Dr. SHAKHASHIRI: I'm glad to hear that. I'm very glad to hear that. So safety is paramount. Enjoying the beautiful booms, you know? I'll just tell you - do we have another minute?

FLATOW: You got about 20 seconds.

(Soundbite of laughter)

Dr. SHAKHASHIRI: Okay. When you watch the fireworks, you know, you see the light before you hear the boom. And that's because light travels much, much faster than sound. And so there are ways by which you can estimate how far the firework is from where you are. And we have that information on my Web site, scifun.org, S-C-I-F-U-N.org.

LICHTMAN: And we linked to it on our Web site.

Dr. SHAKHASHIRI: You have it on your Web site, too.

LICHTMAN: Yep.

FLATOW: Have a happy Fourth, Bassam.

Dr. SHAKHASHIRI: Thank you, Ira. And same to you and yours.

LICHTMAN: Thank you, Dr. Shakhashiri.

Dr. SHAKHASHIRI: Thank you.

LICHTMAN: That was Bassam Shakhashiri, the chemistry professor at the University of Wisconsin, Madison, and the director of the Wisconsin Initiative for Science Literacy.

FLATOW: Thank you, Flora. And our video this week again is?

LICHTMAN: So we have exploding hydrogen bubbles and the science of fireworks.

FLATOW: There you go, on SCIENCE FRIDAY, on our Web site, sciencefriday.com. Thank you, Flora…

LICHTMAN: Thank you.

FLATOW: …for this video pick of the week.

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