If heat is required for a reaction to occur, it is considered an endothermic reaction.

Here's why:

* Endothermic reactions absorb heat from their surroundings. This energy is used to break bonds in the reactants and allow the reaction to proceed. Think of it like needing to add energy to melt ice (which is an endothermic process).

* Exothermic reactions, on the other hand, release heat into their surroundings. They happen spontaneously and often feel warm to the touch.

In summary:

* Endothermic reactions: Heat is absorbed, requiring energy input.

* Exothermic reactions: Heat is released, resulting in energy output.