Chemical reactions can release heat energy or take in heat energy.
Combustion is an Exothermic Reacation - heat's given out
An EXOTHERMIC REACTION is one which GIVES OUT ENERGY to the surroundings, usually in the form of HEAT, which is shown by a RISE IN TEMPRETURE.
In a Endothermic Reaction, heat is taken in
An ENDOTHERMIC REACTION is one which TAKES IN ENERGY from the surroundings, usually in the form of HEAT, which is shown by a FALL IN TEMPRETURE.
Endothermic reactions are less common and less easy to spot. One example is thermal decompisition. Heat must be supplied to cause the compound to decompose, e.g decompotsion of CuCOз.