When heated, most solids expand. If a rod expands, which property changes?

• A. Length increases (linear expansion coefficient)
• B. Density decreases
• C. Mass increases
• D. Volume decreases

Answer: (A)

When a solid is heated, it expands, and the change is most easily described along the dimension you can measure directly—the length. The amount the length changes per degree of temperature change is given by the linear expansion coefficient, α, with the relation ΔL = α L0 ΔT. So as the rod heats, its length increases, which is the most immediate and direct change to observe.

The rod’s cross-sectional area also grows, so its total volume increases as well, and since mass stays the same, the density would drop. But the defining, most straightforward change described by thermal expansion for a rod is the increase in length.