Demonstration

Iron wool is heated in air on a simple ‘see-saw’ balance. The increase in mass is seen clearly.

Lesson organisation

This demonstration takes around 5 minutes once it has been set up.

Apparatus Chemicals

Eye protection

For one demonstration:

Bunsen burner

Heat resistant mat

Wooden metre rule (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

Iron or steel wool, about 4g

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection.

Iron or steel wool, Fe(s) - see CLEAPSS Hazcard. 

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  The change in mass when magnesium burns.

Health & Safety checked, August 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015