This plateau is where a phase change occurs. Likely, this cooling curve observes the changing of a liquid into a solid (so that the temperature is quantifiable). As something cools, it’s losing energy to the surroundings; when a phase change occurs, like liquid to solid, energy isn’t lost directly from the atom, but instead the energy maintained by free motion of the atom is used. This lack of motion reduces it into a lattice (all while maintaining a constant internal energy), finalizing the phase change, after which energy is lost directly from the atom again.
The thing that governs whether a reaction is exothermic is the energy given out / used up to break / form the bonds in the reaction.
<span>When two substances react, the bonds in those substances first break up, releasing energy, before re-forming in a different way, taking in energy. The nature of the bonds that are broken up and reformed determines whether more energy is given out (exothermic) or taken in (endothermic)</span>
