Carbon Disulphide And Nitrous Oxide

The barking dog reaction is a consequence of igniting carbon disulphide (CS2) mixed with nitrous oxide (N2O) – the latter is better known as laughing gas. The reaction generates a bright flash of blueish-purple light and heat, and, more bizarrely, a sound like a dog barking.

Nitrous oxide gas is the source of oxygen – ie the oxidiser – needed to burn the colourless liquid fuel, carbon disulphide. When the reaction takes place in a confined space – such as a long tube – some energy is converted to form the rapid but loud barking noise, due to a fluctuation of pressure. This is an example of a reaction which makes elements from compounds: in this case a yellow coating of sulphur and nitrogen gas are the elements left in its wake.

Carbon disulphide is found in nature as a product of the metabolic processes in plants, and also volcanic eruptions. Nitrous oxide also forms naturally from some species of bacteria, plus through industry and agriculture, and it depletes ozone in the stratosphere.

Used in the distant past as a method of flash photography, the flash it produces is so bright that many people in the photographs would often appear startled. The pervasive smell that sulphur compounds are capable of probably didn’t make it that popular either.

4 steps

1. Energy release – Nitrous oxide reacts with the carbon disulphide releasing energy as heat, expanding the gases.

2. Expansion – As the gases expand, those near the top are forced out of the test tube due to pressure.

3. Differential – The expelled gases lead to a pressure drop within the tube, creating a vacuum-like effect.

4. Bark – As the gases rush back into the tube to balance the pressure, a repeated ‘barking’ noise is made.