Is it possible to to put cold soup from the refrigerator onto a hotplate (without a temperature control) on Shabbat morning to have with lunch or does the soup need to have remained on the hotplate all night if placed there before Shabbat begins?
Please send sources
It is not permitted to place cold soup on the hotplate during Shabbos.
This is considered bishul, because once a liquid has cooled it has lost its virtue of being ‘cooked’, and can be cooked again. [Tihere is an opinion that there is no bishul even for a liquid that was cooked and cooled, but we follow the stringent opinion.]
See Shulchan Aruch, Orach Chaim 318:4 and Rema 318:15.