What method should not be used to heat perchloric acid?

Heating perchloric acid is particularly hazardous due to its highly reactive nature and potential for explosive reactions. The correct answer identifies that gas flames or oil baths should not be used to heat perchloric acid. This is because open flames can provide an uncontrolled heat source, which increases the risk of a violent reaction, especially given that perchloric acid can generate toxic fumes and may react violently with organic materials or reducing agents.

In contrast, other methods such as electric heaters, water baths, and microwave heating can be managed more safely under controlled conditions. Electric heaters can provide a stable and adjustable heat source, while water baths allow for gentle and uniform heating. Microwave heating, when done under appropriate conditions, can also offer a precise and safe method to heat substances without direct flame contact. Therefore, the use of gas flames or oil baths is considered unsafe for heating perchloric acid due to the potential for ignition and uncontrolled reactions.