Rusting (O-Level chemistry)

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RUSTING OF IRON

When iron is left in damp air for some time it become covered with a brown coat called rust. Chemically rust is hydrated iron III oxide Fe₂O₃.xH₂O.

Disadvantage of rusting

1. Weaken objects made of iron
2. Makes objects made of iron look ugly

Conditions necessary for rusting

1. Oxygen not air
2. Water

Experiment to show that oxygen and water is necessary for rusting

The experiment is set up as shown above and left for several days

1. Test tube A contains nails and moist air
2. Test tube B contains nails and dry air because moisture is removed by anhydrous calcium chloride
3. Test tube C contains nail and air free water; boiling removes dissolved air from water while a layer of oil prevents entry of air into water

Observation after several days

1. In test tube A rusting took place because there is both oxygen and water necessary for rusting to take place.
2. In test tube B rusting did not take place because there was not water
3. In test tube C rusting did not take place due to absence of air

Conclusion

Both oxygen and water are necessary for rusting to take place

Method of preventing rusting

1. Keeping iron and steel equipment in air or water free environment, i.e., in the dry places (from water).
2.  Oiling (protects equipment from water and oxygen).
3. Painting (protects from both air and water)
4. Tin plating protects iron from both air and water; however, tin-plate is only effective provided the layer of tin remains intact
5. Galvanizing: this is coating iron with zinc. Zinc protects iron because it is passive in air but also it can reduce iron III to ion.

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