Why does plaster emit warmth after mixing?

Plaster emits warmth after mixing primarily due to an exothermic setting reaction. When the plaster is combined with water, a chemical process occurs where the gypsum (calcium sulfate dihydrate) transforms into calcium sulfate hemihydrate through hydration. This reaction is exothermic, meaning it releases heat as the plaster sets. This heat generation is a result of the energy changes that occur at the molecular level during the formation of the new compounds as the mixture begins to harden.

Understanding this property is essential in various dental practices, as the heat can influence the working time and the setting characteristics of the material. In contrast, the other options do not accurately describe the phenomenon: the chemicals used do not inherently produce heat, plaster does not absorb heat from the environment to produce warmth, and heating plaster beforehand is not necessary for it to set properly.