Which of the following substances is classified as a non-condensable?

A non-condensable substance is one that does not readily change from a gas to a liquid under the temperature and pressure conditions in a refrigeration system. In this context, air fits this classification.

Air consists primarily of nitrogen and oxygen, both of which remain gaseous over the typical operational ranges of refrigerants used in HVAC systems. Non-condensables can lead to inefficiencies in a system, as they do not condense into a liquid and can take up space in the system that would otherwise be occupied by the refrigerant, reducing the system's capacity to cool effectively.

In contrast, the other substances listed—mist or moisture, R-12, and R-134a—can condense under certain conditions. Mist or moisture refers to liquid droplets that can form in a vapor, and both R-12 and R-134a are refrigerants that readily change from liquid to gas and vice versa, depending on the cooling cycle's pressure and temperature. This capability allows refrigerants to absorb and release heat effectively in a refrigeration cycle, making them essential for the cooling process.