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.
Answers
(a) Silver sulphide, copper carbonate coating is formed on Ag & Cu article.
(b) Galvanization is a method of protecting steel and iron from rusting by coating them with a thin layer of zinc. The galvanized article is protected against rusting even if the zinc coating is broken.
(c) Alloy is a homogeneous mixture of two or more metals and non-metals. It is prepared by first melting the primary metal and then dissolving the other elements in it in definite proportion. It is then cooled at room temperature.
(i) On mixing carbon it becomes hard and strong,
(ii) on mixing Ni, and Cr, it become on hard and does
not rust.
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(a) Ag2S (silver sulphide) is formed on silver, basic copper carbonate CuCO3. CU(OH)2 is formed on copper.
(b) The process of coating zinc over iron is called galvanisation. It is used to prevent rusting of iron.
(c) Alloy is a homogeneous mixture of two or more metals. One of them can be non-metal. Alloys are prepared by melting two or more metals together.
(i) Iron does not rust on adding small,quantity of carbon.
(ii) When we form alloy of iron with nickel and chromium, we get stainless steel which is malleable and does not get rusted.