When iron or iron objects are exposed to the moist air for a long time acquires a coating of a brown flaky substance called rust. This process is called rusting.
An activity to investigate the conditions under which the iron rusts is as follows :(i) Take three test tubes A, B and C.
(ii) Pour some water in A and cork it.
(iii) Pour distilled water in test tube B, add about 1‌ml of oil and cork it.
(iv) The oil will float on water and prevent the air from dissolving.
(v) Put some anhydrous calcium chloride in test tube C and cork it.
(vi) Iron nail will rust in test tube A, but they do not rust in test tube B and C.
Conclusion : This can be concluded from the experiment that water is essential for the rusting of iron and iron objects.