Bark Like a Dog (Fun With Chemistry Version)

Blowing things up can be fun — and educational too!

I grew up back in the days when chemistry sets didn’t just include Bunsen burners and glass blowing kits — they were equipped with real chemicals like potassium permanganate, calcium carbide, silver nitrate, salicylic acid, and concentrated acids and bases. I remember mixing potassium nitrate and sugar to make rocket engines, burning my leg with concentrated nitric acid and trying (mostly unsuccessfully) to blow various things up. Unfortunately real chemistry sets went out of fashion after the 60’s when liability concerns forced most of the good stuff (and nearly all the fun and mystery) out of them.

While that nifty Gilbert U-238 Atomic Energy Lab may not be sold at retail anymore, you can still find information on some fascinating, dangerous and politically uncorrect experiments on the interwebs.  Take, for example, the Barking Dog.

Chemists (and alchemists) have been familiar with the exothermic reactions that take place when a mixture of carbon disulfide (CS₂) vapor is ignited in nitric oxide (NO) or nitrous oxide (N₂O) have for centuries.  The Barking Dog demonstration is a specific way of performing of these reactions. The reactant mixture is ignited in a long tube resulting in a nifty bright blue chemiluminescent flash and – if the tube is of the right size – a distinct barking or woofing sound.  The sound is created by a combustion wave that travels down the tube compressing the gas ahead of it — like this:
                (note: fast-forward to 5m30s if you just want to hear the bark)

The Barking Dog can also be created in a long tube covered with a piece of filter paper onto which you’ve placed a bit of white phosphorus ( P₄) coated in carbon disulfide.  As the carbon disulfide diffuses away from the phosphorus and into the filter paper, its vapor saturates the tube and the phosphorus becomes exposed to the air.  The white phosphorus then reacts with atmospheric oxygen and spontaneously combusts, igniting the carbon disulfide vapor in the tube.  If the tube is of the right size – a distinct barking or woofing sound will accompany the chemiluminescent flash.

Carbon disulfide is EXTREMELY flammable in both liquid and vapor forms. It’s dangerous stuff – not just because of its extremely low ignition temperature but also because it can kill you though inhalation or absorption through your skin; so this isn’t a group of experiments you want to try at home.

(Sidebar: in 1873 a German man took out a patent to use the carbon disulfide – nitrous oxide process as a source of light for photography at night.  Although it produced a great flash, the process never caught on, possibly because of the dangers inherent in handling carbon disulfide.)

The Barking Dog is sometimes performed in a line of graduated cylinders of different sizes.  The reactions are set off in series, like some sort of bizarre, exploding, barking pipe organ:

And, properly calibrated, they sound much like the real thing.