Final answer:
The refrigerant that forms in the evaporator after a sudden pressure drop is known as Flash gas, which absorbs heat and provides the refrigeration effect.
Step-by-step explanation:
The name given to the refrigerant that forms in the evaporator due to a sudden drop in pressure between the high side and the low side of the system is Flash gas. This occurs when the high-pressure liquid refrigerant passes through an expansion valve before entering the evaporator. The pressure drop across the valve causes a portion of the liquid refrigerant to rapidly vaporize or "flash" into gas due to the lower pressure in the evaporator. This process is essential in refrigeration cycles as it provides the refrigeration effect by absorbing heat from the environment or the space to be cooled.