Essentially, decomposition reaction are the opposite of combination reactions. A compound decomposes (i.e.,"splits-up") into two or more compounds and/or elements. For example mercury(II) oxide will, upon heating, decompose into mercury metal and oxygen:  Since heat had to be added to make this reaction go, it is an endothermic reaction. Most decomposition reactions are endothermic. Another example of a decomposition reaction is the heating of calcium carbonate (sea shells, chalk):  |
