A chemical reaction creates one or more new substances via starting materials known as "reactants." Scientists classify chemical reactions into types, based on the way the reactants' particles rearrange to produce a new substance. Each type also requires either an input of energy from or a release of energy into its environment for the reaction to take place.
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Types of Reactions
Chemical reactions fall into six main categories, based on the way the atoms have rearranged themselves. In a synthesis reaction, atoms of two or more substances rearrange to form a compound. In decomposition, the opposite of synthesis, a compound breaks down and forms two or more new substances. In single replacement reactions, an element in one compound replaces another element. Double replacement reactions occur when both reactant compounds exchange atoms. A neutralisation reaction -- a type of double replacement reaction -- occurs when an acid and a base react to form a salt and water. In a combustion reaction, a compound reacts with oxygen to form carbon dioxide and water.
Chemical reactions involve the input or output of energy for the breaking and re-forming of bonds. In exothermic reactions, the breaking of the reactants' bonds releases energy, as in the burning of a candle, which gives off light and heat. An endothermic reaction requires energy, usually as heat or light, absorbed from its surrounding. The energy in an endothermic reaction, much like a reactant, must be used to produce the products. Without it, the reaction cannot take place.
Conversation of Mass
The law of conservation of mass states that when a chemical reaction takes place, the total mass of the products equals the mass of the reactants. A chemical reaction neither creates nor destroys atoms -- thus, no mass is lost or gained.
Sodium and chlorine provide an example of a chemical reaction, as they undergo an extreme chemical change when brought together. Individually, they can be dangerous; for example, chlorine gas used in World War I as a chemical weapon caused damage in the respiratory tract once exposed and inhaled. But, when sodium and chlorine react, the atoms rearrange to form sodium chloride, better known as table salt, which has a new set of harmless chemical properties.
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