The surface of copper powder becomes coated with which of the following substance when reacted with oxygen?

Copper oxide

The correct answer is option 1. Copper oxide.

When copper powder reacts with oxygen in the air, a chemical reaction occurs to form copper oxide (CuO). This process is known as oxidation.

The chemical equation for the reaction is:

\[ \text{2Cu} (s) + \text{O}_2 (g) \rightarrow \text{2CuO} (s) \]

In this reaction:

Copper (Cu) atoms from the surface of the copper powder react with oxygen molecules (\(O_2\)) from the air.

Each copper atom combines with one oxygen molecule to form a molecule of copper oxide (\(CuO\)).

This process continues until the entire surface of the copper powder is coated with a layer of copper oxide.

The formation of copper oxide on the surface of copper is a common occurrence when copper is exposed to air for an extended period. The copper oxide layer appears as a dark or dull coating on the surface of the copper, giving it a characteristic color. This process is responsible for the gradual tarnishing or darkening of copper objects over time.

The formation of copper oxide provides a protective layer that helps prevent further oxidation of the underlying copper metal. This layer acts as a barrier, preventing oxygen and moisture from reaching the underlying copper surface and slowing down the rate of corrosion. However, if the copper oxide layer is disturbed or removed, the underlying copper metal can continue to oxidize.

Overall, the reaction of copper powder with oxygen to form copper oxide is a common example of an oxidation reaction and is responsible for the characteristic appearance of oxidized copper surfaces.