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. So in our hypothetical example, we have reactant AB, breaks down or decomposes into two new products, A plus B. Now let's look at a very common example. Now, the hydrolysis of water or the decomposition of water.