What does the term "thermal expansion" refer to in steam systems?

The term "thermal expansion" refers to the phenomenon where the volume of a substance increases as it is heated. In the context of steam systems, as steam temperature rises, the kinetic energy of the water molecules increases, causing them to move apart and occupy a greater volume. This is essential for steam systems, as the efficiency of steam generation and transportation relies on the understanding that as water transitions to steam, its volume expands significantly.

Option C accurately captures this concept, stating that there is an increase in the volume of steam as it heats up. Understanding thermal expansion is crucial for engineers when designing systems that handle steam, as they must account for this change in volume to prevent issues such as pressure build-up or equipment failure. This principle is foundational in many areas of engineering, particularly in systems using heated fluids.