To ensure that you understand the material in this chapter, you should review the meanings of the following bold terms in the following summary and ask yourself how they relate to the topics in the chapter. Scientific laws are general statements that apply to a wide variety of circumstances. One important law in chemistry is the law of conservation of matter , which states that in any closed system, the amount of matter stays constant. Chemical equations are used to represent chemical reactions . Reactants change chemically into products . The law of conservation of matter requires that a proper chemical equation be balanced . Coefficients are used to show the relative numbers of reactant and product molecules. In stoichiometry , quantities of reactants and/or products can be related to each other using the balanced chemical equation. The coefficients in a balanced chemical reaction are used to devise the proper ratios that relate the number of molecules of one substance to the number of molecules of another substance. Chemical reactions can be classified by type. Combination reactions (also called composition reactions ) make a substance from other substances. Decomposition reactions break one substance down into multiple substances. Combustion reactions combine molecular oxygen with the atoms of another reactant. Oxidation reactions are reactions in which an atom loses an electron. Reduction reactions are reactions in which an atom gains an electron. These two processes always occur together, so they are collectively referred to as oxidation-reduction (or redox ) reactions . The species being oxidized it called the reducing agent , while the species being reduced is the oxidizing agent . Alternate definitions of oxidation and reduction focus on the gain or loss of oxygen atoms, or the loss or gain of hydrogen atoms. Redox reactions are easily balanced if the overall reaction is first separated into half reactions , which are individually balanced. Oxidation-reduction reactions are common in organic and biological chemistry. Respiration , the process by which we inhale and metabolize oxygen, is a series of redox reactions. In the absence of oxygen, redox reactions still occur in a process called anaerobic metabolism . Antioxidants such as ascorbic acid also play a part in the human diet, acting as reducing agents in various biochemical reactions. Photosynthesis , the process by which plants convert water and carbon dioxide to glucose, is also based on redox reactions. Template:HideTOC