And that means evaporation is a physical reaction. Now, on the contrary, whenever a compound goes from one molecular formula to a different compound with a different molecular formula, this type of reaction is known as a chemical reaction. Now, examples include combustion of hydrocarbons in which a hydrocarbon burns in the presence of oxygen to produce carbon dioxide and water. Other examples include oxidation reduction reactions, combination and addition reactions. Now, let's look at the following chemical reaction in which the following hydrocarbon C, two S, H, six reacts in the presence of oxygen to form carbon dioxide and a water molecule. Now, remember what the conservation of energy and the conservation of mass tells us.