Final answer:
During the setting up of gypsum, an exothermic reaction occurs, releasing heat and causing the material to harden. This process increases the temperature of the surroundings.
Step-by-step explanation:
When gypsum sets, it undergoes an exothermic reaction, which means it releases heat as it hardens. This heat release is typical of many plaster-based materials as they change from a more fluid to a solid state, like in the hardening of plaster containing slaked lime, represented by the chemical equation Ca(OH)2 + CO2.
An exothermic process can raise the temperature of the surroundings, in contrast to an endothermic process which absorbs heat and cools the surroundings.