@anon140790 - It's safe to microwave food in polystyrene. The material doesn't contain water and it is the liquid molecules that are affected by the microwaves.
It might melt because it's in contact with the heating food but that's it. And it's been tested by the FDA to make sure that it's not going to be harmful.
Individual containers might have a mix of different kinds of plastics though, so if it doesn't specifically say on the container that it is microwave safe, then you're better off just switching to a ceramic plate.