Instant hot and cold packs are found in many common stores and work when two substances react. Depending on the substances that react, the reaction either absorbs heat or gives off heat.The enthalpy of a hot pack is because the reaction .

A hot pack becomes warm because its reaction transfers heat to the surroundings. A reaction that gives off heat is exothermic, so the enthalpy change is negative. Therefore, the correct choice is “gives off heat.”

AI-written and checked against the verified answer.