Final answer:
The most likely difference between the two pieces of a copper bar cut into different sizes is their physical size and mass, whereas the elemental properties of copper remain the same.
Step-by-step explanation:
When a teacher cuts a bar of copper into two different-sized pieces, the most likely difference between the two pieces is their physical size and mass. The composition of the copper remains the same because cutting a substance does not affect its elemental properties. However, because the pieces are different sizes, the larger piece will have more mass and volume, while the smaller piece will have less. This is analogous to cutting magnets, where each piece still has a north and south pole regardless of the cut; in the same way, the copper's properties are uniform throughout the bar.