The causes of rusting
In this class experiment students put iron nails in various conditions including wet, dry, air-free and salty to find out what causes iron to rust.
This experiment will need to be set up in one lesson and then left for more than 3 days before being re-examined. It could be left set up for longer if necessary.
Iron nails, 4
Anhydrous calcium chloride granules (IRRITANT) Students with sensitive skin should be offered gloves.
Boiled deionised water (15 min boil) (Note 1)
Sodium chloride (table salt)
Refer to Health & Safety and Technical notes section below for additional information.
Per pair or group of students:
Rubber bung to fit test tube
Pen or other means of labelling test tubes
Health & Safety and Technical notes
Wear eye protection.
Anydrous calcium chloride, CaCl2(s), (IRRITANT) - see CLEAPSS Hazcard.
Sodium chloride, NaCl(s) - see CLEAPSS Hazcard.
1 It is best if the deionised water is boiled eg in a kettle, as close to the start of the lesson as possible and supplied warm to the students. They could boil it themselves for 15 minutes in a beaker on a Bunsen burner, but whether this is advisable will depend on the class.
a Label the test tubes 1-4.
b About ¼ fill tube 1 with deionised water and add a nail.
c About ¼ fill tube 2 with boiled deionised water and add a nail. Carefully pour a little oil over the surface to prevent air from reaching the water.
d Mix some salt with some deionised water to make a solution. About ¼ fill tube 3 with this mixture and add a nail.
e Put a nail into tube 4 and add about 2 cm depth of anhydrous calcium chloride granules. These absorb water. Put a bung in this tube to prevent any further water from getting in.
f Leave for at least 3 days and then note any changes in appearance of the nails.
You could ask students to tabulate which conditions are present or absent in each of the tubes.
- Tube 1 – water and air
- Tube 2 – water but no air (it is removed during boiling and the oil prevents any extra from dissolving in the water and reaching the nail)
- Tube 3 – water, air and salt
- Tube 4 – air, no water (the calcium chloride removes the water from the air and the bung prevents any extra from entering.)
They should see that the nails in tubes 2 and 4 do not rust. The nail in tube 3 rusts the most. From this they should be able to conclude that water and air (actually oxygen in the air) are essential for rusting. Salt can increase the rate of rusting. This can lead to a discussion about rust protection and methods which can be used to keep air and water away from the iron such as paint, grease and plastic coating.
Very simply, rusting is the reaction of iron with oxygen – but water is an important part of the process too. For fuller details of the reactions, see the website below.
Health & Safety checked, April 2008
Page last updated on 31 July 2012