The combustion of iron wool
Iron wool is heated in air on a simple ‘see-saw’ balance. The increase in mass is seen clearly.
Read our standard health & safety guidance
Lesson Organisation
This demonstration takes around 5 minutes once it has been set up.
Apparatus and Chemicals
For one demonstration:
Eye protection
Bunsen burner
Heat resistant mat
Wooden metre rule (see note 1)
Aluminium cooking foil, about 10 cm x 10 cm
Retort stand, boss and clamp
Plasticine, few grams
Knife edge, triangular block or something similar
Steel wool (Low hazard), about 4g
Technical notes
Steel wool (Low hazard) Refer to CLEAPSS Hazcard 55A
1 A shallow groove cut across the width of the ruler at the 50 cm mark will help when balancing it on the knife edge. Cover the end of the meter ruler with foil to protect it from the Bunsen burner.
Procedure
a Cover one end of the meter ruler with foil to protect it from the Bunsen burner. Take about 4 g of steel wool and tease it out so that the air can get around it easily. Use a few of the strands to attach it to the end of the ruler.
b Balance the ruler on a knife edge or triangular block at the 50 cm mark. Weight the empty end with plasticine until this end is just down (see the diagram). This part is critical.
c Place a heat resistant mat underneath the steel wool.
d Wear eye protection. Light the Bunsen burner and heat the steel wool from the top with a roaring flame. It will glow and some pieces of burning wool will drop onto the heat resistant mat. Heat for about a minute by which time the meter ruler will have over-balanced so that the iron wool side is down.
Teaching notes
As you are setting up, ask the students whether they think the iron wool will go up, down or remain the same. Many will predict a weight loss.
If fine steel or iron wool is used then it may be possible to light it using a splint.
Equation:
Iron + oxygen → iron oxide
2Fe(s) + 3/2 O2(g) → Fe2O3(s)
You may also wish to look at experiment Change in mass when magnesium burns.
Health & Safety checked, April 2008
Updated 10 Apr 2008Average rating: 
3 out of 5
Your reviews
It seems like the ruler will be charred. I would like to have a better way or device to oxidize the iron.
I have MRE (Iron and NaOH) heaters: add water, produces heat and gives off hydrogen. Should lose mass, to show students that something lighter than air can still have mass.
Submitted by: Joseph T. Peterka on 18 December 2008
If you're worried about naked flames you should be able to ignite the wire wool using a low voltage power supply - and this means you can also set up a switch mechanism so that you can provide some separation between young scientists and the apparatus.
Submitted by: Richard on 18 March 2009
I have not tried this demonstration. But as a additional problem, I believe the final mass of the Iron oxide could be measured, then used to determine the molecular formula for iron oxide.
Submitted by: Todd Nore on 27 November 2009
You should write why it weighs more.
Submitted by: Delila on 18 December 2008