Corrosion

    The rusting of iron (iron oxide), tarnishing of silver (silver sulphide), development of green coating on copper (copper carbonate) and bronze are some of the examples of corrosion.

• Do you know why corrosion occurs?

Let us find

Activity 2



• Take three test tubes and place clean iron nails in each of them.

• Label these test tubes A,B and C. pour some water in test tube A and cork it.

• Pour boiled distilled water in test tube B, add about 1 ml of oil and cork it. The oil will float on water and prevent the air from dissolving in the water.

• Put some anhydrous calcium chloride in test tube C and cork it. Anhydrous calcium chloride will absorb the moisture, if any, from the air. Leave these test tubes for a few days and then observe (see figure).



    You will observe that iron nails rust in test tube A, but they do not rust in test tubes B and C. In the test tube A, the nails are exposed to both air and water. In the test tube B, the nails are exposed to only water, and the nails in test tube C are exposed to dry air.


• What does this tell us about the conditions under which iron articles rust?

    In metallic corrosion, a metal is oxidised by loss of electrons generally to oxygen and results in the formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air.


The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. During corrosion at a particular spot on the surface of an object made of iron, oxidation takes place and that spot behaves as anode. We can write the reaction as.

Anode:2Fe(s) → 2Fe2+ + 4e-


page no:247

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