An unknown substance undergoes a chemical change that gives off heat. what can you tell me about this substance?

Explanation:

A chemical reaction in which heat is absorbed by the reactant molecules is known as an endothermic reaction.

For example,

On the other hand, a chemical reaction in which heat energy is released by the reactant molecules is known as an exothermic reaction.

For example,

Thus, we can conclude that when an unknown substance undergoes a chemical change that gives off heat then it means earlier the substance has more heat energy but after the chemical change there occurs a decrease in its heat energy because some of the heat has been released by it.