Page 212 - Chemistry ICSE Class X

198 ICSE Chemistry – 10
by electroplating iron object with nickel or chromium. Nickel and
ŝŶĐ ŝƐ ŵŽƌĞ ĞůĞĐƚƌŽƉŽƐŝƟǀĞ ƚŚĂŶ ŝƌŽŶ͘ EJTQOKWO EQCVKPIU UJKPG CPF IKXG C FGEQTCVKXG ſPKUJ /QUV QH VJG
^Ž͕ ĂƐ ůŽŶŐ ĂƐ njŝŶĐ ŝƐ ƚŚĞƌĞ ŽŶ ĂŶLJ ƉĂƌƚ
ŽĨ ƚŚĞ ŐĂůǀĂŶŝƐĞĚ ŝƌŽŶ͕ ŝƌŽŶ ĚŽĞƐ ŶŽƚ DCVJTQQO ſVVKPIU E[ENG RCTVU ECT DWORGT FQQT JCPFNGU GVE CTG
ŐĞƚ ĐŽƌƌŽĚĞĚ͘ dŚĞ ŐĂůǀĂŶŝƐĞĚ ŝƌŽŶ ŝƐ chrome-plated.
ƵƐĞĚ ĨŽƌ ŵĂŬŝŶŐ ƌŽŽĨ ƐŚĞĞƚƐ͕ ǁĂƚĞƌ ƒ By tinning. Tin (Sn) is also not attacked by air and water. Tin is
ƚĂŶŬƐ͕ ďƵĐŬĞƚƐ͕ ĞƚĐ͘
nontoxic. So, the containers used in the packaging of food items are
coated with tin. Coating of an iron object with a thin layer of tin is
called tinning.
Iron sheet coated with tin is called tin plate. Tin plates are used for
making packaging containers for oils, ghee, milk powder, pickles and
other food items.
Tin is less electropositive than iron. So, if tin coating on iron gets
scratched, the exposed iron starts getting corroded at that place very
rapidly. So, tin protects iron as long as the tin coating remains intact.

ƒ By alloying. Iron can be prevented from corrosion by alloying
iron with chromium and nickel. Stainless steel is an alloy of iron
containing iron 74%, chromium 18%; and nickel 8%. Stainless steel
does not corrode. Stainless steel is used for making cutlery, knives,
domestic utensils, etc.
ƒ By connecting iron to a more electropositive metal. Rusting of iron
can be prevented by connecting iron object to a highly electropositive
metal, such as magnesium. Magnesium, being more electropositive,
Iron pillar near Qutab Minar at acts as anode, while iron acts as cathode. Magnesium does not allow
Mehrauli in Delhi (400 BC). The the oxidation of iron and thus, protects it from rusting.
iron pillar is 8 m high and weighs This method, called cathodic protection is generally used for the
6 tonnes (6000 kg). This pillar still protection of underground pipes and tanks from corrosion.
stands without showing any sign of
rusting. Cathodic protection of an underground pipeline is shown in Fig. 7.11.

Fig. 7.11 Cathodic protection of iron by connecting
Galvanized steel tubes
it to magnesium or any other more electropositive metal
Illustrative Questions

Q.1. Zinc is more electropositive than iron. So, it should get corroded faster than iron. But, it does not happen. Instead
it is used for protecting iron (galvanisation). Explain.
Ans. Zinc when exposed to moist air gets covered with a protective coating of basic zinc carbonate, (Zn(OH) .ZnCO ).
2
3
This coating prevents zinc from further reaction. Thus, zinc does not get corroded, and can be used for protecting
iron from rusting.
Q.2. Why is iron galvanised with zinc? Can it be galvanised with copper? If not, why?
Ans. Iron is galvanised with zinc to protect it from rusting. This is because zinc is more electropositive than iron.
Copper is less electropositive than iron. Therefore, iron cannot be galvanised with copper (to prevent rusting).
Q.3. Explain why the surface of some metals acquires a dull appearance when exposed to air for a long time.