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Question
State the change in colour observed in following case mentioning the reason:
Copper powder is strongly heated in air.
Solution
Copper powder turns from brown to black when heated vigorously in the presence of oxygen because oxygen reacts with copper powder to form copper oxide, which has a dark colour. Thus, the appearance of copper oxide is what causes the colour change.
\[\ce{2Cu(brown) +O2 ->2CuO(black)}\]
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