And note that this triangle on top of the arrow means that heat must be added for our reaction to occur. That's what this triangle means. It doesn't mean change. It means energy heat. Now let's look at a very common second type of chemical reaction known as decomposition reactions. Now, decomposition reactions are essentially the reverse of combination reactions, where in combination reactions, two reactants combine in decomposition reactions, a reactant, decomposes or dissociates into two or more new compounds.