Final answer:
The most time-efficient phase for recovering refrigerant from a system is the liquid phase, as it requires less energy for the phase transition and takes advantage of the exothermic properties of condensation.
Step-by-step explanation:
To efficiently recover refrigerant from a system, it is best to remove as much as possible in the liquid phase. This is because substances like refrigerants require less energy to transition from liquid to gas than from solid to gas. During phase changes where a refrigerant changes from gas to liquid, as occurs in the condenser component of an air conditioner, the transition is exothermic, meaning it releases heat. Refrigeration systems take advantage of this property to cool spaces by having the refrigerant absorb heat when it vaporizes (absorbing heat from the environment), then releasing that heat when it condenses back to a liquid. Since gases are the most energetic state of matter and solids are the least, it is more time-efficient to recover refrigerant in its liquid state rather than as a gas.