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When solid copper metal is heated in presence of atmospheric oxygen, it turns black. This is due to the formation of copper (II) oxide or cupric oxide. The reaction can be represented by:
`2 Cu (s) + O_2 (g) -> 2 CuO (s)`
In terms of ions, the equation can be written as:
`Cu -> Cu^(2+) + 2e^-`
`O_2 +4e^(-) -> 2O^(2-)`
This is an oxidation-reduction reaction. Copper gets oxidized by losing 2 electrons and in the process its oxidation number changes from 0 (in copper metal) to +2 (in copper oxide). In comparison, oxygen gets reduced and its oxidation number changes from 0 (in oxygen gas) to -2 (in copper oxide). The loss and gain of 2 electrons by the reacting species (copper and oxygen) makes this an ionic equation.
The reaction can be reversed by putting the black cupric oxide in a hydrogen atmosphere, which will convert the oxide back to copper metal and also generate water.
Hope this helps.
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