The chemical formula of Plaster of Paris is CaSO₄·½H₂O. Gypsum (CaSO₄·2H₂O) is heated at 373 K (100°C) to obtain Plaster of Paris.
Examiners expect both the chemical formula and the exact temperature. The formula is sometimes written as CaSO₄·½H₂O or (CaSO₄)₂·H₂O — both are accepted. The temperature 373 K is the key fact; writing only "100°C" is also acceptable. Since this is 1 mark, one concise line covering both parts is sufficient.