A glass rod and a copper rod are each subjected to increasing tensile forces. The glass rod snaps suddenly without any permanent change in shape. The copper rod changes shape permanently before eventually breaking. Describe the difference in behaviour between the two rods and use the terms brittle and ductile in your answer.

Glass is a brittle material – it breaks suddenly without any permanent change in shape. The glass rod snaps at or very near the elastic limit, so it fractures without plastic deformation.
Copper is a ductile material – it can be permanently stretched or drawn into a new shape before breaking. The copper rod undergoes plastic deformation, changing shape permanently before it finally fractures.
Both materials obey Hooke’s law up to the elastic limit: they behave elastically and return to their original shape if the force is removed below that point. However, glass fractures at or near this limit, whereas copper extends further, undergoing plastic deformation beyond the elastic limit before breaking.