Question

How do you know if a reaction is endothermic?

Answer 1

In an endothermic reaction heat is used up.
So you would know it is endothermic if there is less heat after the reaction than the heat before the reaction.
Or you could just see if there is a heat use up

Answer 2

Endothermic is when heat is absorbed, so like when ice turns into water, it absorbed heat and energy from the air because now the molecules in the ice started moving faster and more freely to become a liquid state.