See the explosion that sounds like a barking dog

The famous Barking Dog Experiment combines a few chemicals in a tube and causes explosions that sound, most people say, like barking dogs. I say they sound a little more like phasers, or other ray guns. Learn a little chemistry, listen to a few explosions, and judge for yourself.

One of the unfortunate things about chemistry is sometimes, when chemicals get together, they explode. This has lead to a lot of lost lives, and not a little janitorial overtime.

To chemists' credit, they've been able to turn some of the more immediate and violent reactions into funny little experiments. One of the most well-known is the Barking Dog experiment. It starts with some heat, a little nitrous oxide (two nitrogen atoms bonded with an oxygen) and a bit of carbon disulphide (a carbon atom getting pulled between two sulfur atoms). The carbon and sulfur (both atoms that know a good thing when they see it) grab hold of two oxygen atoms each, separating them from the nitrogen, and make off with them. It's an explosive reaction, giving off a lot of heat and light. At the end of it all traumatized nitrogen atoms cling together in pairs, forming a gas, and some leftover powdery sulfur, which didn't get in there fast enough to grab any oxygen for itself, presses itself to the walls of the tube and sulks.

But when you listen to the video, you don't hear the loud boom that accompanies most explosions. You hear a rough but tonal sound that depends on the length of the tube. The tube makes music, and music is made by vibrations. How does the reaction vibrate?

As it turns out, this reaction can be increased and decreased by pressure. But not in a direct one-to-one relationship. Instead, the reaction rate will increase as a cube of the increase in pressure. Increasing the pressure by three will increase the rate of the reaction by twenty-seven.

So when the pressure on the reaction increases slightly, the reaction explodes massively. Exploding means expanding. Usually, when something expands in a tube, it will expand until the pressure inside and outside are equal. But because the explosion is so proportionately massive, it pushes gasses out of the tube until the pressure inside the tube is actually a little under the pressure outside the tube. The explosion has created a relative vacuum inside the tube, so air goes shooting back inside. This kicks up the reaction again, and it blows its top, pushing out more gas, and getting itself under general air pressure once again. Back in goes the air. Back out it's blown. And the entire tube ends up vibrating and making a sound.

But you be the judge of the sound — doesn't it sound more like a ray gun?

Top Image: U.S. Air Force Photo by Josh Plueger

Via The Naked Scientists.