What happens to copper when it is annealed?

When copper is annealed, it undergoes a heat treatment process that results in a significant change in its physical properties. During annealing, copper is heated to a specific temperature and then allowed to cool slowly. This process removes the internal stresses that have accumulated in the metal due to processes such as work hardening, where the metal becomes harder and more brittle through mechanical deformation.

As a result of annealing, the crystalline structure of the copper is altered, leading to a reduction in hardness and an increase in ductility, which means the metal can be shaped or drawn into various forms more easily. This softening effect is particularly important for applications where flexibility and ease of manipulation are desired, such as in plumbing and electrical connections where copper is often used.

The other options describe effects that do not occur during the annealing process. Hardening would imply an increase in hardness, while turning black or green pertains to oxidation or weathering processes rather than the annealing process itself.