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
A. negative
B. positive
C. zero
because the reaction
A. absorbs heat
B. gives off heat
C. does not change temperature

the answer is 0.42 is that is an option. this question provides no info

answerto predict;

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.