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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.


17 Responses to The Barking Dog

  1. Mark Davis

    Your Brother is a giant

  2. Dan Science

    It’s true.

  3. Joan Davis

    You guys are great, the kids will love this experiment.


    I love you experiments.

  5. Meacha Cameo

    You guys sooo cool. I like for real all the cool experiments.

  6. Kirsi

    how do you make n2o? we are really struggling

    • Dan Science

      Hi there,

      We buy our nitrous oxide, but it’s fairly easy to make. Two ways I know of off the top of my head:
      1) Thermal decomposition of ammonium nitrate: NH4NO3 → 2H2O + N2O
      This reaction happens at a little under 200 degrees Celsius, but take great care to not overheat it as this can be dangerous! Ammonium nitrate can be found in some types of instant cold packs.

      2) Combining and heating sulfamic and nitric acids: HNO3 + NH2SO3H → N2O + H2SO4 + H2O
      Sulfamic acid is used in many cleaning products, and can sometimes be bought relatively pure from the store.

      Hope that helps!

  7. William

    how do you make carbon disulfide?
    I can only find over priced stores online and no actual informative info.


    • Dan Science

      Carbon disulfide is made from union of the elements. This requires a very high temperature furnace, and is quite hard to do in an amateur setting. I know of only one home chemist that has accomplished it! The general idea is to boil elemental sulfur and pass the vapor over finely powdered carbon at several thousand degrees, then collect and condense the CS2 vapor formed. Not an easy task!

  8. como ganhar dinheiro

    congratulations for the excellent text

  9. Alejandra

    The products of the chemical reaction are correct but one of the chemical reagents is not. I did this experiment in my laboratory and the nitrous oxide that you really need is NO (a colourless gas too).
    with in contact with O2 (g) reats and form NO2 wich is red/brown gas.

    • Dan Science

      Well I’ve done this reaction many, many times using nitrous oxide (N2O) as the reactant. That’s the one that is specified in all that literature related to this classic demonstration. Nitric oxide (NO) probably will also work since it too is an oxidizer, as you’ve done in your lab. I now do a version of this demo that uses methanol in place of carbon disulfide. So there are certainly several different variations on the experiment, none of which are really “incorrect.”

  10. Alejandra

    Thank you for the information. It’s really interesting to know that, because now, i have more new questions about this reaction. Kind regards, A science lover too.

  11. abcde

    Is the above chemical equation correct? Will SO2 be produced?
    I saw some websites suggest this:8 N2O + 4 CS2 → S8 + 4 CO2 + 8 N2

    • Dan Science

      SO2 is definitely produced. Our sore throats can attest to that! Here’s a more detailed rundown of the chemistry:
      In the case of excess N2O there is complete combustion:
      CS2 (g) + 6N2O (g) → CO2 (g) + 2SO2 (g) + 6N2 (g)
      When there is excess CS2, incomplete combustion results in the formation of elemental sulfur that coats the walls of the container:
      CS2 (g) + (6 – 2n)N2O (g) → CO2 (g) + (2-n)SO2 (g) + nS (s) + (6-2n)N2 (g)
      The value of n depends on the strength of the excess amount (0 ≤ n ≤ 2).

  12. Kimberly M

    when doing the barking dog, what are the amounts you would need with zinc and hydrochloric acid?
    what would the concentration be for the hydrochloric acid?

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