Iron nails are placed in five environments:

• Tube 1: Just in air
• Tube 2: A part of the nail is immerged in water
• Tube 3: A part of the nail is immerged in salt water
• Tube 4:The nail is completely immerged in boiling water and a layer of oil separates the water from the air
• Tube 5: A dehydration product (Calcium chloride or Copper Sulfate) absorbes the humidity contained in the air.

The different hypotheses or this experiment allow us to explain the formation of rust:

Only the nails in the three tubes on the left rust, at a fast rate. The two experiments on the right slow down or stop the formation of rust.

This allows us to confirm that iron does not rust until it is in contact with air and water.