Question

The Statue of Liberty is made up of a metal. Over a period of time, the metal's colour has changed and the statue gives a greenish look. Answer the following questions:

(A) What is corrosion? (B) Why do silver articles turn black after some time? (C) Copper reacts with moist gas 'A' present in the air and slowly loses its shiny surface. It turns green. Identify 'A' and the substance of green coating formed on the surface of copper.

OR

(C) What is an alloy? How is an alloy made?

Accepted Answer

(A) Degradation of metal by atmospheric gases/moisture. (B) Silver sulphide formation. (C) Gas A is CO2; green coating is basic copper carbonate.. (A) **Corrosion:** The slow process of decay and eating away of metals when exposed to atmospheric moisture, air, and chemicals. (B) **Silver tarnish:** Silver reacts with $H_2S$ gas in air to form a black coating of Silver Sulphide ($Ag_2S$). (C) Gas 'A' is **Carbon dioxide ($CO_2$)**. The green coating is **Basic Copper Carbonate** [$CuCO_3\cdot Cu(OH)_2$]. **OR (C):** **Alloy:** A homogeneous mixture of two or more metals, or a metal and a non-metal. **Method of Preparation:** Prepared by melting the primary metal first, then dissolving other elements in definite proportions, followed by cooling to room temperature.

Corrosion of Metals and Alloys

Explanation

Practise this under board exam conditions