Final answer:
A heating mantle must be plugged into a VARIAC for precise control of the voltage and current, allowing gradual adjustment and safe operation of the equipment's temperature, which is not possible when plugged directly into a wall outlet.
Step-by-step explanation:
The heating mantle should be plugged into the VARIAC and not directly into the wall outlet to allow for precise control of the voltage and current supplied to the heating mantle. Direct connection to a wall outlet would deliver a fixed voltage, which could lead to overheating or underheating, depending on the reaction requirements. The VARIAC, which stands for Variable Auto-transformer, can regulate the power supplied to the heating mantle, providing a way to finely adjust the temperature to the needed level for a chemical reaction.
In addition to this control function, a VARIAC can protect the heating element by ramping up the voltage slowly, preventing sudden electrical surges that could damage the heating mantle. Considering electrical safety, the three-wire system with an earth/ground wire offers protection from potential electrical hazards, whereas plugging directly into an older two-wire system without a ground may pose a risk of shock, especially if the insulation is worn. Accordingly, grounding and other safety features are essential when dealing with high-powered laboratory equipment such as heating mantles.