Which refrigerant is known to sublimate?

The correct choice is CO2, which is known to sublime under certain conditions. Sublimation is the process where a substance transitions directly from a solid to a gas without passing through the liquid state. In the case of CO2, at atmospheric pressure, it exists as a solid (dry ice) and sublimates directly into carbon dioxide gas as it warms up, rather than melting into a liquid first.

This property is particularly important in various applications, such as refrigeration, where solid CO2 can be utilized for cooling purposes without the complications associated with liquid phases. The sublimation of CO2 makes it unique among refrigerants, especially in applications requiring low temperatures.

Other refrigerants listed, like ammonia, R-134a, and Freon 12, do not exhibit sublimation under normal operating conditions. Instead, they exist as liquids at lower temperatures and pressures and undergo traditional phase changes between solid, liquid, and gas states, meaning they would melt into a liquid before transitioning to a gaseous phase when heated. Understanding the behavior of refrigerants under different conditions is crucial for effective system design and operation.