Final answer:
Silver tarnishes due to hydrogen sulfide, and tin coating on copper vessels prevents corrosion and extends durability.
Explanation:
(a) Silver earrings darken after a period of time: Silver tarnishes in the presence of substances like hydrogen sulfide in the air, forming a layer of silver sulfide which gives the metal a darker appearance.
(b) Copper vessels are coated with tin to increase their lifespan: Coating copper vessels with tin helps prevent corrosion and extends their durability by creating a barrier between the copper and the surrounding environment, reducing the chances of oxidation.
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