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 $1\,\text{ml}$ 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 $\mathrm{C}$ and cork it.(vi) Iron nail will rust in test tube A, but they do not rust in test tube $\mathrm{B}$ and $\mathrm{C}$.Conclusion : This can be concluded from the experiment that water is essential for the rusting of iron and iron objects.