Combustible Powders

Description

A combustible powder is injected into a closed vessel that contains a burning candle. An explosion ensues.

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• A combustible powder burns in air. The more finely divided the powder is, the greater the reacting surface, and the faster the reaction. Lycopodium powder sprayed as an aerosol into a flame burns brilliantly.
• When a combustible powder is confined as a dust in a closed space, the reaction may be explosive.

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Students should wear goggles in anticipation of the flying can cover. Avoid contact (particularly inhalation) with the lycopodium. Students allergic to lycopodium should leave.

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1. Light a Bunsen burner. Hold the squeeze bottle containing combustible powder near the flame, and squeeze a burst of powder into the flame.
2. Place one-half teaspoonful of lycopodium powder on a watch glass. Heat in a Bunsen flame. After considerable heating, the powder chars but does not catch fire.
3. Set up a safety shield.
4. Light a match. Ignite two or three of the wicks on the candle. Insert the lighted candle into the coffee can.
5. Work quickly; the heat from the candles will melt and burn the plastic can cover. Cover the can.
6. Invert the squeeze bottle to put a "charge" of powder in the nozzle. Insert the nozzle into the hole in the side of the can. Squeeze.
7. Note evidence of reaction.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 048 Combustible Powders

• A combustible powder burns in air. The more finely divided the powder is, the greater the reacting surface, and the faster the reaction. Lycopodium powder sprayed as an aerosol into a flame burns brilliantly.
• When a combustible powder is confined as a dust in a closed space, the reaction may be explosive.
• Identify several materials that might be used as combustible powders.
• Grain dust and coal dust explosions are all too often encountered in spite of many safety practices employed to avoid them. Every year, several workers die from these explosions. Suggest some measures that should be used to minimize the danger from these explosions.

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Purpose

To illustrate the explosive reaction of a finely divided combustible powder.

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• safety shield
• 3-lb coffee can with 8 mm hole drilled in side 5 cm above bottom
• 3 candles of different height connected by wax
• matches
• combustible powder (lycopodium, flour)
• squeeze bottle with adjustable nozzle neck (ketchup, mustard container)
• Bunsen burner, tubing
• watch glass
• tongs

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• Lycopodium powder is the combustible powder of choice. Keep the powder dry by storing the lycopodium in a well-sealed bottle, and place this in a sturdy plastic sack. Close the sack with a twist-tie. Dry other powders such as flour in a microwave oven prior to use.
• The purpose of this demonstration requires that the teacher demonstrate that the aerosol burns brilliantly, and then that the same reaction confined to a closed space may be explosive.
• Use a safety shield.

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Teacher preparation: 15 minutes

Presentation: 10 minutes

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• The flying cover may cause damage.
• Burning chemicals may cause damage.
• Some individuals are allergic to lycopodium.

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• Review the location of the fire extinguisher. Check that the fire extinguisher is fully charged. Wear goggles in anticipation of the flying can cover. Avoid contact (particularly inhalation) with the lycopodium.
• Use a safety shield.

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Save all materials except used matches for reuse. Dispose of used matches with ordinary trash.

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Presentation Question:

• Identify several materials that might be used as combustible powders.
  • Flour and very fine saw dust are possibilities. (Powdered metals should never be used; these are extremely hazardous due to very high heats of reaction.)

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Explosions in grain elevators are explained on the basis of the phenomena illustrated during this demonstration. Grain dust and coal dust explosions are all too often encountered in spite of many safety practices employed to avoid them. Safety practices include schemes for keeping the solid aerosols from forming and schemes for preventing sparks.

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• combustible powder
• explosive
• combustion
• explosion
• dust explosion
• reaction rate
• surface area
• heterogeneous reaction

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