Chemical reactions conservation of mass oxidation particles
Age Group
11 12 13 14
Expected time
1 period
Chemical seesaw.PDF
beaker symbol

Chemical seesaw

What’s the mystery? A balance is set up with two pieces of iron wool. One piece is set alight: it appears to be getting smaller but, mysteriously, the balance tips the other way.
Domain(s) Chemistry
Subdomain keywords Chemical reactions, particles, conservation of mass, oxidation.
Age group 11 to 14 years old.
Expected time for the mystery Approximate time for teacher preparation: 45 min.Approximate time in classroom: one 50-min lesson.
Safety/supervision Be careful when burning the materials in the classroom. Use eye protection.
Preparation and list of materials. For teacher demonstrations:

  • Simple balance (for instructions, see the resources section).
  • Four aluminium trays (two can be stuck on top of the metre ruler). When burning the paper, place two more trays on top and remove these so you have clean trays to burn the iron wool.
  • Two pieces of screwed-up paper of equal mass
  • Two pieces of iron wool of equal mass. Clean these in acetone first to remove any grease.
  • Bunsen burner
  • Wooden splint
  • Two pieces of magnesium strip
  • Tongs
  • Gas jar of pure oxygen


  • Piece of carbon (charcoal)
  • Deflagrating spoon
Learning objectives Students will construct explanations using particles to demonstrate what happens during a chemical reaction.
Resources Instructions on how to build the balance equipment: