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(a)WRITE THE CHEMICAL NAME OF THE COATING FORMS ON SILVER COPPER ARTICLES WHEN THESE ARE EXPOSED TO MOIST AIR. (b)EXPLAIN GALVANISATION.
Copper reacts with moist air to form (greenish )copper oxide and silver reacts with moist air to form (black )silver oxide .
Galvanisation is the process of applying a protecting zinc coating to steel or iron inorder to prevent rusting .
   The process of galvanisation consists of the following steps :
(i)Cleaning of iron sheets : The iron particles to be galvanised are first cleaned by means     of  a sand blast .The sheets are then dipped in dilute sulphuric acid solution to remove any oxide film and washed with water.This is known as pickling .
(ii) Galvanising  : In this step zinc metal is deposited on the cleaned iron sheets .Following methods are employed for this purpose .
 (a) Sherardizing : The articles made of iron are dipped into zinc dust in a revolving drum at a temperature just below the melting point of zinc .After keeping therefore some time it is taken out and heated to 800 0c when an alloy of iron and zinc is formed at this temperature at the surface .
(ii) Metallizing : Molten zinc is sprayed on the cleaned iron particles and an adherent coat of zinc metal is obtained .
(ii) Electro -galvanising : Iron articles are continuosly passed through a zinc plating bath of sodium zinc tetra cyanide .The article is made the cathode when metallic zinc is deposited on it.The deposit so obtained on the article is very smooth,shining and durable .






      


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