Surface tension on the water in a steam drum is increased by what?

The correct answer centers on the role of impurities on the water surface in a steam drum. Surface tension is influenced by the presence of substances on a liquid's surface which can either increase or decrease this property. Impurities that float—such as oils or other contaminants—tend to create a film that stabilizes the surface and can effectively increase surface tension.

When impurities are present, the molecules at the surface behave differently compared to pure water. These contaminants can disrupt the polar interactions amongst the water molecules, leading to a rearrangement of these molecules at the surface, which ultimately can result in an increase in surface tension.

In contrast, the other options do not directly relate to an increase in surface tension. For example, water temperature can actually reduce surface tension because higher temperatures increase molecular movement and reduce the cohesive forces among water molecules. High pressure does not significantly affect the surface tension in a manner that would be directly comparable to the effect of floating impurities and adequate ventilation typically pertains to air circulation rather than affecting surface tension in water.

Focusing on the role of impurities helps clarify why they play a determinant role in modifying the surface tension within the steam drum, leading to potential impacts on the operation of steam systems.