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Let's write the chemical equation for a reaction that takes place while you cook on a gas stove top. Oxygen (O2) from the air reacts with natural gas (CH4) to produce carbon dioxide gas (CO2) and water (H2O). In this reaction, O2 and CH4 are the reactants since they are “used up” by the reaction. Carbon dioxide and water are the products. In words, the reaction can be written as below. For now, you can ignore the coefficients that appear in front of O2 and H2O. 
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The chemical equation is the most accurate and least ambiguous way to describe the reaction. The equation tells exactly what changes to what, and the relationships between the quantities of each which will be used and produced. When natural gas burns completely, one mole of methane (CH4) combines with two moles of molecular oxygen (O2) to produce one mole of carbon dioxide (CO2) and two moles of water (H2O). You don't see the water because it is vapor at the temperature of this reaction.
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You may be wondering when to use the term "chemical equation" instead of "chemical reaction." A chemical reaction is the process of chemical change where reactants become rearranged into new products. A chemical equation is simply the representation of a chemical reaction as a series of symbols. You can call this series of symbols either a chemical equation or a chemical reaction: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g), but if you were watching the piece of solid magnesium produce hydrogen gas bubbles as it reacted with the aqueous hydrochlroic acid, it would be incorrect to say "look at that chemical equation!" You are observing a chemical reaction, whose chemical equation is written as: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). 
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Does the chemical equation CH4 + 2O2 → CO2 + 2H2O give a complete representation of the burning of natural gas? When we look at a fire, we see that fire gives off light and we feel that fire gives off heat. So, if the chemical equation does accurately represent the phenomenon of burning then where is the heat and where is the light? A more complete accounting of the reaction, including energy, is later in the chapter. Energy can be either a reactant or a product. In the case of methane, the energy is a product because it is given off by the reaction.
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