That's good safe thinking! So, I don't consider it a stupid question.
The Ferric chloride will heat just fine in your Microwave, provided that the container isn't metallic. I do it all of the time
Although, if the solution is brought to a boil, you're in deep trouble with your Mom. Warm soup temperature works good for me.
I suppose you can go all out, and resort to the Ferric Chloride data sheet, or possibly MSDS to find the "optimum" temperature.
One point, I'd leave the door open for a few hours to allow any fumes/vapor to escape before you go popping your vegetable soup in the microwave. :wink:
If you or your Mom is still leary about heating in the microwave, you can fill the sink with hot water, and place your container in the water to warm the solution. Be advised that the solution will etch a stainless sink and leave ugly spots, so rinse any accidents (Spills, splashes) off immediately.
Good luck!