Exploding Flour Tin

A miniature silo explosion .

 

 

 

Category - Chemicals & Reactions

Key Idea - Combustion

 

Purpose -

A fantastic way to show the following concepts -

 

- the variable nature of a flammable substance in different conditions

- the increasing rate of reaction with a greater surface area

- the forces created by holding a reaction under pressure

 

Nature -  Demonstration
Materials -  Tin with a metal sealing lid (e.g. baby formula tin), hammer, metal punch, a piece of thin metal sheet, a drinking straw, candle stub, taper, matches, flour, spoon or spatula
Method - 
  1. Glue the small piece of metal sheet to form a shelf that comes out from the side of the tin. It needs to be down towards the bottom at about the level the the candle flame will burn.
 
  1. Knock a hole in the side of the tin that will snugly fit the drinking straw.  The hole needs to be located just above the level of the shelf.
  2. Pile up some flour on the shelf, and push the straw through the hole so that when you blow the flour will puff up as a cloud through the tin.
  3. Place a candle in the bottom of the tin and light with the taper.
  4. Jam the lid on the top of the tin, and before the candle has a chance to go give a strong quick blow through the drinking straw.
  5. With luck the flour will ignite and blow the lid off the tin, accompanied with a whoosh of flame.
Safety - Keep clear of the lid as it flies upwards.  
Explanation - Flour is an unusual substance because it requires a lot of air, and a great deal of surface area before it will ignite.  Once it has both of these though, it is quite explosive.
Notes - It is best to complete steps 1 and 2 before class and leave the apparatus set up for quick use in future.

I usually make a small pile of flour on the bench first, and try to light it to show the low flammability of flour before going on to steps 3 to 6.  This is good fun to act really casual about so that when it does go off, it comes as a surprise to the class. 

A variation of this is to set up the hose and funnel to blow across a candle flame outside of the tin so you can see a flame-thrower effect.

Occasionally flour silos will explode under similar, though larger, circumstances