Smoke Bombs

Smoke bombs are a way cool experiment to wow the kids and neighbors. They really should be called smoke and fire bombs.

How To Make Smoke Bombs

  1. Mix 1/2 c. saltpetre (found at some drug stores, call around to see who has it) with 1/2 c. sugar in a sauce pan. Heat over medium to high heat, stirring constantly, until sugar melts. You will have a stiff liquid.
  2. Pour into a paper cup, carefully. It’s hot!
  3. Let it cool for a bit, then stick several wooden matches down in the cooling sugar mixture, with the match heads up.
  4. Let it cool completely.
  5. Set your smoke bomb outside on a sidewalk or driveway, where it can’t catch anything on fire. It’s cool to do this at night or in the evening, when the light is low. Light the matches and the paper cup. It will smolder for awhile and then take off, producing a spectacular flame and lots and lots of smoke.

The sugar, containing lots of carbon, burns fabulously. Burning is one form of oxidation, the process where oxygen is added to a chemical.

The saltpetre, potassium nitrate, facilitates the burning and produces the solid, which rises into the air = smoke.

Here’s the reaction

potassium nitrate + sugar —> nitrogen gas + potassium carbonate + carbon dioxide + water

Using chemical symbols

KNO3 + C12H22O11 —-> N2 + K2CO3 + CO2 + H2O

(note that this chemical equation is not balanced according to moles in the reaction. If you don’t know what that means, don’t worry about it.)

Additional Layers

  • Find out about saltpetre, where does it come from? Who first invented it? What did they use it for? Why do drugstores today carry it?
  • Learn about the history of matches. Who invented them? What’s the difference between a “strike anywhere” and a “safety match”?
  • What is smoke?
  • Why is the chemical symbol for potassium a “K”?
  • Try pouring 1/2 cup of sugar into a cup by itself and lighting it on fire. What happens?
  • Potassium and other alkali metals by themselves are extremely reactive. Why are some elements more reactive than others? Completed and fulfilled atoms always have 8 electrons in their outer shell, the outermost level of electrons. Alkali metals, like potassium and sodium, are so reactive because they have just one electron in their outer shell, the easiest way for them to become stable is to lose that one electron. Sounds like no big deal, right? Forces on the atomic scale are, well, massive. Think atom bombs for instance. When a pure alkali metal reacts with moisture, even just water vapor in the air it usually explodes violently. In this experiment the potassium is combined with nitrogen, which fulfills potassium’s electron needs and so makes a stable compound, until you burn it anyway.
Here’s a diagram of a Potassium atom, with that one electron in its outermost shell. Image by Greg Robson, CC license, Wikimedia.

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