Final answer:
The statement is false; most solids expand when heated due to an increase in the motion of atoms or molecules, taking up more space. Water is an exception, expanding when it freezes. Gases have the largest coefficients of volume expansion, expanding more rapidly with temperature changes than solids or liquids.
Step-by-step explanation:
The statement 'most solids do not expand when they are heated' is false. The vast majority of solids expand when heated. This is because heating typically increases the energy of the atoms or molecules within a solid, which causes them to vibrate more vigorously. As a result, they tend to occupy more space, and the solid expands. This condition is valid as long as the solid is isotropic, meaning it expands uniformly in all directions without a preferred direction, maintaining its shape but increasing in size. However, it's important to note that water is an exceptional case that actually expands upon freezing, which is quite different from other materials.
When discussing thermal expansion, the coefficients of thermal expansion play a significant role. For most materials, an increase in temperature increases the solid's size by a specific fraction in each dimension, which in turn can lead to substantial forces if constrained. For example, if a solid that is expanding upon heating is confined, it may exert enough force to deform or break its container. This principle applies not only to solids but also to liquids, and even more so to gases, which tend to have the largest coefficients of volume expansion and therefore expand and contract quite rapidly with changes in temperature.