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The Barking Dog

Posted by on March 3, 2011

This has to be my favorite experiment I’ve done so far! The Barking Dog is a really exciting demonstration of a combustion reaction, or a chemical reaction where one material burns in another. This demonstration very beautifully brings together many of the other topics Bill and I talk about in the show, like energy, light, sound, and color!

In the Barking Dog reaction, a mixture of two gases is placed in a long glass tube that is sealed at both ends with rubber stoppers. When ready, the tube is stood upright, one of the stoppers is removed, and the mixture is lit at the top with a match. A wave of flame quickly moves down the tube as the gases burn, speeding up the farther down it is. The flame is bright blue because of the sulfur that is produced in the chemical reaction (sulfur burns with a blue color), and the rapidly expanding gases shoot out of the top of the tube with a loud “Woosh” that sort of sounds like a dog’s bark!

The video is a clip from our first live show, held at the STEM Community Forum on March 1st

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The Science:

This reaction happens between two chemical compounds: nitrous oxide (chemical symbol: N2O) and carbon disulfide (CS2). We fill the tube with the N2O first, which is an invisible gas like air. You might know nitrous oxide better by it’s more common name: laughing gas! They use this at the dentists office as an anesthetic, something to make your body numb so you don’t feel it while they work on your teeth.

After that, we add a very tiny amount of the carbon disulfide. This is a liquid, but very quickly evaporates into a gas when it’s placed in the tube. This material is very flammable and burns easily, allowing us to light the mixture with a match or lighter. When ignited, we get a very vigorous reaction that produces a lot of heat, light, and sound as the initial chemicals are converted into different ones:
CS2 (g) + 4N2O (g) → CO2 (g) + SO2 (g) + S (s) + 4N2 (g)
The letters in parentheses in the equation tell you what phase the chemical is in:

(s) = solid
(l) = liquid
(g) = gas

From the equation, you can see that this reaction produces elemental sulfur (chemical symbol: S)! That’s why at the end of the reaction you see the tube has a light dusting of yellow on the inside of it – that’s the sulfur that was made! The rest of the products escape as invisible gases.

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