Sunday 15 July 2012

Science: Hot Iron

This is a very sparkly experiment.  For this experiment I do suggest you wear your safety goggles and ensure that there is a responsible adult present.

Heat an iron nail up using a Bunsen burner or an alcohol burner what do you notice?  It goes black right?  This is Iron oxide. 

Now heat up some iron filings...

To do this experiment you will need:

  • A bunsen burner or (like ourselves) an alcohol burner
  • We used a metal baking tray to protect our work surface.
  • Iron filings
  • An iron nail
  • Tongs
  • An old metal spoon
  • Matches or a lighter

First grab an iron nail in your tongs.  Remember once heated you must not touch the nail!  Put it into the flame, or even better just on top of the flame and watch it heat up.  What happens?  It goes black right?

Now, put some metal filings on your spoon and very carefully tilt your spoon just enough to see the sparks fly!

Do you want to know why the iron filings sparked like a firework but the iron nail didn't? Well the iron filings give off sparks when they burn in air, converting from iron to iron oxide.  But the reason why it’s different to the iron nail is because of surface to volume ratio.

In order for something to burn, the surface in contact with the air must be heated to its ignition temperature.  It is then that the surface can react with oxygen in the air, giving off heat and by-products of the reaction.  

As you attempt to heat a surface to reaction temperature, heat is conducted away from the surface, partially preventing you getting the effect you want.  The mass of a small particle is such that the entire particle can be heated to ignition temperature hence it sparks!

The rate of the reaction between iron and air is largely proportional to surface area of the iron exposed to air.  The same is true with most combustible solids.  A small wood match is easy to light and keep burning.  A large, single log is difficult to burn.

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