Black snake (firework)
"Black snake" is a type of consumer firework consisting of a chemical that when set on fire burns with an intumescent reaction, exuding a long twisting string of ash resembling a snake. They are sold as pellets which are placed on a noncombustible surface and ignited with a match, burning slowly with little or no flame. Two earlier traditional formulas are now banned in retail fireworks because of the toxic chemicals they contain: the Pharaoh's Serpent which contains mercury (II) thiocyanate and the sugar snake which contains potassium dichromate. Sometimes these are performed as do-it-yourself amateur chemistry demonstrations.
Pharaoh's snake
[file:Weze faraona.png|thumb|Pharaoh's serpent demonstration]The Pharaoh's snake is a more dramatic experiment and it requires more safety precautions than the sugar snake due to the presence of toxic mercury vapor and other mercury compounds.
This reaction was discovered by Friedrich Wöhler in 1821, soon after the first synthesis of mercury thiocyanate. It was described as "winding out from itself at the same time worm-like processes, to many times its former bulk, of a very light material of the color of graphite." For some time, a firework product called "Pharaoschlangen" was available to the public in Germany but was eventually banned when the toxic properties of the product were discovered through the deaths of several children who had mistakenly consumed the resulting solid.
The Pharaoh's snake experiment is conducted in the same manner as the sugar snake experiment, however, the former uses mercury(II) thiocyanate instead of powdered sugar with baking soda. This must be done in a fume hood because all mercury compounds are hazardous.
After igniting the reagents, mercury thiocyanate breaks down to form mercury(II) sulfide, carbon disulfide, and carbon nitride. Graphitic carbon nitride, a pale yellow solid, is the main component of the ash.
Carbon disulfide ignites into carbon dioxide and sulfur(IV) oxide.
While carbon nitride will break down into nitrogen gas and cyanogen.
When mercury(II) sulfide reacts with oxygen, it will form gray mercury vapor and sulfur dioxide. If the reaction is performed inside a container, a gray film of mercury coating on its inner surface can be observed.
Sugar snake
[file:Black snake experiment.webm|thumb|Black snake experiment]Unlike the carbon snake, which involves the reaction of sulfuric acid instead of sodium bicarbonate, the sugar snake grows relatively faster and to a significantly larger volume.
Solid fuel is used in this experiment. The solid fuel can be sand that is sufficiently covered in ethanol or hexamethylenetetramine. A white mixture of sucrose and sodium bicarbonate will eventually turn black and the snake will grow about long after it is lit.
Three chemical reactions occur when the snake is lit. Sodium bicarbonate breaks down into sodium carbonate, water vapor, and carbon dioxide:
Burning sucrose or ethanol produces carbon dioxide gas and water vapor:
Some of the sucrose does not burn, but merely decomposes at the high temperature, giving off elemental carbon and water vapor:
The carbon in the reaction makes the snake black. The overall process is exothermic enough that the water produced in the reactions is vaporized. This steam, in addition to the carbon dioxide product, makes the snake lightweight and airy and allows it to grow to a large size from a comparably small amount of starting material.