Q34 of 400 Page 192

Explain why, the surface of some metals acquires a dull appearance when exposed to air for a long time.

The surface of some metals acquires a dull appearance when exposed to air for a long time because oxide layer is formed on the surface of metals as they react with moisture, air and any other gases.

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33 Which one of the methods given in column I is applied for the extraction of each of the metals given in column II:

Column I


(i) Electrolytic reduction


(ii) Reduction with Carbon


(iii) Reduction with Aluminium


Column II


(a) Aluminium


(b) Zinc


(c) Sodium


(d) Iron


(e) Manganese


(f) Tin

 34 Give reason why copper is used to make hot water tanks but steel (an alloy of iron) is not. 35 Why does aluminium not corrode right through? 35 What is meant by 'anodising'? Why is it done?