$(a)$ Write the chemical name of the coating that forms on silver and copper articles when these are left exposed to moist air.
$(b)$ Explain what is galvanisation? What purpose is served by it?
$(c)$ Define an alloy. How are alloys prepared? How do the properties of iron change when:
$(i)$ small quantity of carbon,
$(ii)$ nickel and chromium are mixed with it.

(A-D) The coating on silver is silver sulphide $(Ag_2S)$ and on copper is basic copper carbonate $(CuCO_3 \cdot Cu(OH)_2)$.
$(b)$ Galvanisation is a method of protecting steel and iron from rusting by coating them with a thin layer of zinc. The purpose is to prevent corrosion,as the zinc layer acts as a sacrificial anode and protects the underlying metal even if the coating is scratched.
$(c)$ An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal. It is prepared by melting the primary metal and then dissolving the other elements in it in a definite proportion,followed by cooling to room temperature.
$(i)$ When a small quantity of carbon is mixed with iron,it becomes hard and strong.
$(ii)$ When nickel and chromium are mixed with iron,it becomes stainless steel,which is hard and does not rust.