When R-12 is superheated and enters a reciprocating compressor, it leaves as:

When R-12 is superheated and enters a reciprocating compressor, it leaves as a superheated gas. This outcome is due to the nature of how refrigerants work in a vapor-compression refrigeration cycle.

In the superheating process, R-12, which is already in a vapor state, receives additional heat after it has evaporated but before it enters the compressor. This heat input raises the temperature of the vapor above its saturation temperature, ensuring that the R-12 is entirely in a gaseous state and has an increased temperature and energy level.

As the superheated gas enters the compressor, it is compressed, which further increases its temperature and pressure. The superheated condition is crucial because it prevents the refrigerant from condensing within the compressor, which could cause damage and decrease efficiency.

Other choices reflect states of matter or conditions that are not applicable to R-12 after it has gained superheat from its previous state as a vapor. Therefore, the correct identification of R-12 leaving the compressor as a superheated gas highlights the effective transition of refrigerants throughout the refrigeration cycle, emphasizing the importance of maintaining proper thermal conditions for optimal compressor operation and system efficiency.