Question: Can I place chicken on a hotplate on Shabbos to heat it up?
Answer: Chazal (Shabbos 36b) prohibited reheating even cooked food on Shabbos. According to the Ran (quoted by the Mishna Berura 253:55), the reason is because it looks like cooking, while Rabbenu Tam (quoted in Shaar Hatziyun 253:37) writes that it is to prevent people from stoking or adjusting the flame on Shabbos.
The Shulchan Aruch (OC 253:5) allows one to heat up a roasted food on top of another pot on a stove on Shabbos, as this does not resemble cooking (See Magen Avraham OC 318:26). The Pri Megadim (quoted by the Biur Halacha 253:3) writes that this only applies if the pot has food in. Thus, one would only be able to heat food if there was already a pot on the stove (See Chazon Ish 37:9).
Nonetheless, R’ Shlomo Zalman Auerbach (quoted in Shemiras Shabbos Kehilchasa 3:1n112) and R’ Shmuel Wosner (Shevet Halevi 1:91) hold that even the Pri Megadim would agree that one may use an empty pot on a hotplate. Thus, one may place an empty pot or upturned foil container on the hotplate to place solid food on top.
R’ Shlomo Zalman Auerbach (Meor Hashabbos 10:4) writes that hot plates should be treated the same as regular stoves and one who wishes to heat up food on Shabbos must place it on top of another pot.
Nonetheless, R’ Ovadia Yosef (Yechave Daas 2:45) and R’ Eliezer Waldenberg (Tzitz Eliezer 8:26:5) allow one to place fully cooked solid food directly onto a hot plate on Shabbos providing that the hotplate can’t be used to cook and has no knobs to adjust the temperature (See Igros Moshe OC 1:93; 4:74:35; Shemiras Shabbos Kehilchasa 1:n71).In conclusion, one may heat up dry food (even if it contains a little sauce). However, any liquid food such as chicken in gravy must be on the hotplate before Shabbos.