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What is meant by ‘rusting of iron’? With the help of labelled diagrams, describe an activity to find out the conditions under which iron rusts.
When an iron object is left in damp air (or water) for a considerable time, it gets covered with a red-brown flaky substance called rust. This is called the rusting of iron.
Experimental set-up
- In test-tube A containing an iron nail, we put unboiled water so that about two-thirds of the nail is immersed in water and the rest is above the water, exposed to damp air. In this way, the iron nail in the third test-tube has been placed in air and water together.
- In tube B containing iron nail, we put boiled distilled water. Boiled water does not contain any dissolved air (or oxygen) in it (this is because the process of boiling removes all the dissolved air from it). A layer of oil is put over boiled water in the test-tube to prevent the outside air from mixing with boiled water. In this way, the iron nail in the second test-tube is kept in air-free boiled water.
- In tube C containing iron nail, we put some anhydrous calcium chloride and close its mouth with a tight cork. Anhydrous calcium chloride absorbs water moisture from the damp air present in the test-tube and makes it dry. In this way, the iron nail in the first test-tube is kept in dry air (having no water vapour in it).
The mouth of all three test tubes is closed with a cork and it is kept aside for about one week.
After a week, we observe the iron nails kept in all three test-tubes, one by one
Observation and Inference:
(i) No rust is seen on the surface of iron nail kept in dry air (water-free air). This tells us that the rusting of iron does not take place in the air alone.
(ii) No rust is seen on the surface of the iron nail kept in air-free, boiled water in the second test-tube. This tells us that rusting of iron does not take place in water alone.
(iii) Red-brown rust is seen on the surface of iron nail kept in the presence of both air and water together. This tells us that the rusting of iron takes place in the presence of both air and water together.
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