In a lab, Brett is mixing hydrochloric acid with water. This is an extremely explosive mixture that releases heat. What kind of reaction is this, endothermic or exothermic reaction

The definition of an exothermic reaction is releasing heat and endothermic reaction is an absorption of heat.This reaction is exothermic because it releases heat and doesn't absorb it when mixing hydrochloric acid with water. 

Exothermic because you are releasing heat not absorbing.