What does "thermal expansion" refer to in plumbing systems?

Thermal expansion refers to the phenomenon where materials expand in volume when they are heated. In plumbing systems, this is particularly relevant when discussing water because as water is heated, its temperature rises, causing the molecules to move more energetically and occupy a larger space. This increase in volume can create pressure within pipes and fixtures, which is why plumbing systems must account for thermal expansion.

In this context, it’s crucial for plumbers to consider how heated water will affect the system when designing and installing piping. Proper allowances, such as expansion joints or flexible fittings, are important to accommodate this increased volume and prevent stress or damage to the plumbing infrastructure.

Other options do not accurately define thermal expansion. For instance, the first choice discusses pressure changes while cooling, which is not relevant to the concept of expansion. The third option relates to the behavior of water when frozen, and while it's an interesting aspect of water's properties, it does not pertain to thermal expansion in the context of heating. Lastly, while metal pipes do expand when heated, the correct definition focuses on the volume increase in water.