Silver articles turn black when kept in the open for longer time due t

Silver articles turn black when kept in the open for longer time due to the formation of

[amp_mcq option1=”H₂S” option2=”AgS” option3=”AgSO₄” option4=”Ag₂S” correct=”option4″]

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UPSC CDS-2 – 2020

Silver articles tarnish and turn black when exposed to the air for a prolonged period. This tarnishing is primarily due to the reaction of silver (Ag) with hydrogen sulfide (H₂S) gas present in the atmosphere (often produced by the decomposition of organic matter or industrial pollution). This reaction forms a layer of black silver sulfide (Ag₂S) on the surface of the silver article. The overall reaction is 2Ag + H₂S → Ag₂S + H₂.

Silver tarnish is caused by the formation of silver sulfide (Ag₂S) through the reaction of silver with hydrogen sulfide in the air.

H₂S is the reactant from the air. AgS is not the correct chemical formula for silver sulfide (silver has a +1 charge, sulfide has a -2 charge, so Ag₂S is formed). AgSO₄ is silver sulfate, which would typically be formed by a reaction with sulfuric acid or sulfates, not the common cause of black tarnish in air. The black deposit is definitively silver sulfide (Ag₂S).

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