Thursday, May 13, 2010

Negative X: Start a Fire With Water


A very popular chemistry demo. This combination of chemicals produces a highly water sensitive mixture that instantly ignites when a drop of water lands on it.


When a drop of water touches a small pile of this mixture, it instantly bursts into a blue-green flame, accompanied by a large cloud of smoke. This is an old formula we used to make when we were kids back in the early 1970s. If I recall, we made several small heaps and tried to hit each one with a squirt gun. We used to call it "Negative-X" (I have no idea why), but I'm sure the it's known by other names. It's a great attention grabbing classroom chemical demonstration.

"NEGATIVE-X"
Ammonium Nitrate 
14.0 grams
Ammonium Chloride
1.5 grams
Zinc Dust
34.5 grams

Comments:
  • Grind the Ammonium Nitrate and the Ammonium Chloride together in a mortar & pestle until they are a fine powder. Now pay close attention.... once the remaining chemical, Zinc Dust, is added, this composition will become extremely water sensitive. In fact, if it's humid outside or your chemicals have a high moisture content, the mixture may ignite at any time.
  • It is suggested that before you add the required amount of Zinc Dust, you move your experiment outdoors just in case accidental ignition occurs. As always, gloves and eye protection are absolutely called for.
  • Take about a teaspoon full of the mixture and make a small pile of it on the ground. Make sure the ground is dry and that there are no flammable materials nearby (dry grass, paper, etc.).
  • Press a little indentation into the top of the pile and add a drop of water.

QUICKLY STAND BACK!


In a second or so it will bubble, hiss and smoke, and instantly burst into a blue-green flame.

WARNINGS

You cannot store this mixture. It will eventually absorb enough water from the air to ignite on its own. Use all that you make each time. Never make large quantities at one time. Do not make this mixture on a rainy or humid day. It can spontaneously ignite at any time.

Also note that as soon as all the components are mixed, it will become extremely water sensitive and may spontaneously ignite without warning! Even a tiny amount of water will ignite the mixture. You must use this composition immediately after it is made. Do not mix it with other chemicals. It's recommended to only mix this formula & conduct this experiment outside.


Here's the chemical reaction that takes place:

  1. Cl- (from NH4CL) acts as a catalyst on the decomposition of NH4NO3:

Cl-

NH4NO3-----------------> N2O(g) + 2 (H2O)(aq)

  1. Water in the reaction causes the decomposition of more NH4NO3, which is an autocatalytic effect.
  2. The reaction melts the NH4NO3 and allows the oxidation of the zinc. The overall reaction is:

Zn (s) + NH4NO3 (s)------> N2 (g) + ZnO (s) + 2 (H2O) (g)

- United Nuclear

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