Take three glass bottles with wide mouths. Label them A,B and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water which has been boiled for several minutes, to the same level as in A. In bottle C, take the same boiled water and of the amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C so it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.

The nails in bottles B rust a little, nails in A are the most rusted and that in C remain unchanged.

For rusting both water and oxygen are necessary. Both the factors are present in the bottle A since oxygen is dissolved in water.

In bottle B, water is boiled and hence dissolved air is removed. Due to lack of oxygen, iron nails do not rust much.

In bottle C, the layer of oil present prevents dissolving of atmospheric air in the water and hence no rusting occurs.