In an exothermic reaction, heat can be considered a

Answer is: product of reaction.
There are two types of reaction: endothermic reaction (chemical reaction that absorbs more energy than it releases) and exothermic reaction (chemical reaction that releases more energy than it absorbs). 
According to Le Chatelier's principle when the reaction is exothermic heat is included as a product.

In an exothermic reaction, heat can be considered a product while in an endothermic reaction heat can be considered a reactant.
Exothermic reaction is a chemical reaction that releases energy in form of light or heat to the surroundings. The total energy of the products is less than the total energy of the reactants, thus the change in enthalpy is negative. Endothermic reactions on the other hand are chemical reactions in which heat is absorbed from the surroundings. 

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The answer is nitrogen-containing bases