Practical: Investigating the Rusting of Iron

Rusting is an oxidation process and a redox reaction. Iron atoms lose electrons to form $Fe^{3+}$ ions (oxidation), while oxygen is reduced. This reaction forms a flaky, orange-brown solid called hydrated iron(III) oxide. The word "hydrated" means the structure contains water of crystallisation (water molecules chemically bound within the solid crystal). Because the iron reacts with both oxygen and water to form this new compound, the mass of the iron object increases as it rusts. Unlike aluminium, which forms a protective non-porous layer when it oxidises, rust flakes away to expose fresh iron underneath, leading to continuous destruction.

Investigating the Conditions for Rusting

To prove that iron requires BOTH oxygen and water to rust, you must set up an experiment isolating these variables over a period of about one week.

First, clean three non-galvanised iron nails with sandpaper to remove any pre-existing oxide layers. Then, set up three separate test tubes to test different conditions.

Tube 1 (Control - Air and Water):

• Place an iron nail in the test tube.
• Add regular water and leave the tube open to the air.
• This exposes the nail to both oxygen and water.

Tube 2 (Excluding Oxygen):

• Place an iron nail in the test tube.
• Add water that has been fiercely boiled. Boiling the water removes any dissolved oxygen.
• Finally, add a layer of oil on top. This acts as a physical barrier method, preventing oxygen from the air from re-dissolving into the water.

Tube 3 (Excluding Water):

• Place an iron nail in the test tube.
• Add granules of anhydrous calcium chloride. This chemical acts as a desiccant (a drying agent) to absorb any moisture or water vapour from the air in the tube.
• Seal the test tube firmly with a rubber bung to prevent fresh, moist air from entering. A cotton wool plug can be used to separate the nail from the chemical, which is an irritant.

Results and Equations

After leaving the three test tubes for roughly a week (keeping temperature and nail size as control variables), you will observe the following results:

• The nail in Tube 1 will be coated in an orange-brown, flaky rust.
• The nails in Tube 2 and Tube 3 will remain completely shiny and unchanged.
• This visually demonstrates that if either oxygen or water is missing, rusting cannot occur.

The rate of rusting in Tube 1 could be further increased by adding sodium chloride (salt) or making the conditions acidic.

The overall reaction for rusting can be summarised by the following word equation:

$$\text{Iron} + \text{Oxygen} + \text{Water} \rightarrow \text{Hydrated iron(III) oxide}$$

The balanced symbol equation uses an $x$ to represent the variable number of water molecules in the hydrated compound:

$$4Fe(s) + 3O_2(g) + 2xH_2O(l) \rightarrow 2Fe_2O_3 \cdot xH_2O(s)$$