In which situation would you expect steam density to be lowest?

In examining steam density, it is important to understand the relationship between temperature and the state of water transitioning into steam. Steam density is defined as the mass of steam per unit volume and is directly influenced by both temperature and pressure.

When steam is at extremely high temperatures, particularly well above its boiling point, it occupies a larger volume due to increased kinetic energy of the molecules. As the temperature of the steam increases, the molecular spacing increases, leading to lower steam density. Therefore, steam density tends to be lowest at higher temperatures because the molecules are moving more rapidly and are spaced further apart.

In contrast, when water is heating prior to boiling, the water is still in a liquid state and much denser than steam. As the temperature is at boiling point, the density of steam is typically higher than at extremely elevated temperatures. When the boiler is off and cooling, steam would start to condense back into water, resulting in an increase in density as the steam turns back into the denser liquid state.

Thus, at extremely high temperatures, the density of steam is indeed lowest due to the expansion of the gas, confirming the reasoning behind this choice.