GrammarWhat is the complete subject in the sentence To change a liquid to a solid heat must be taken away.
Grammatically speaking, this is not a well written sentence. What you're really talking about is the change. It might be better written like this:
Changing a liquid into a solid requires the removal of the heat source.
Changing then becomes your subject.
The complete subject is everything before the verb. In this case, it is "To change a liquid to a solid heat". However, the simple subject is "heat".