Phase changes in matter are either endothermic or exothermic. Endothermic reactions require heat to form new chemical bonds, while exothermic reactions create more energy than is needed, producing heat. Exothermic reactions are used in matches, campfires, and fireworks. Scientists use a calorimeter to measure heat produced by reactions. Forensic scientists study exothermic reactions to better understand fires and explosions.
Phase changes in matter are divided into two basic categories: endothermic and exothermic. Exothermic means “external heat” in Greek, as opposed to “internal heat” of endothermic. Endothermic reactions require heat, or energy, to form new chemical bonds, such as when an ice cube is melted into liquid and then boiled into vapor. Exothermic reactions, on the other hand, create more energy than is needed to facilitate them, raising the temperature immediately around the reaction. This happens when rain forms into a cloud and also when rain freezes into ice on the ground.
Some of the most illustrative exothermic reactions involve flames. Matches, campfires, fireworks, and rockets use these reactions to produce energy that is used for a variety of purposes. Many facilitate endothermic reactions such as making bread, cooking soup, or warming your hands. However, exothermic reactions don’t just give off heat; they often require at least an initial supply of energy that provides what is termed activation energy. In exothermic reactions, on the other hand, the energy invested will always be less than the total energy produced; the spark will never be bigger than the flame.
Other reactions of this type are more difficult to experience. When rust forms on metal, another common exothermic reaction, the process does not appear to give off heat. The heat has been released, however; for several hours, in the right conditions of humidity, light and temperature, the mixture of iron and oxygen produces iron oxide and a minimum of heat.
Scientists use an instrument called a calorimeter to measure the amount of heat produced by a phase change or chemical reaction. This involves running the reaction in a water bath to measure the temperature change produced. The results indicate whether a given reaction is endothermic or exothermic.
Chemistry teachers often employ one or more carefully controlled experiments to illustrate the principles of exothermic reactions. The lesson can be as simple as striking a match and explaining how a spark ignites an exothermic reaction between sulfur and oxygen to produce heat. Another common experiment involves mixing water and calcium chloride, an antifreeze agent also used in portable heating packs, then measuring the amount of heat produced by the reaction.
Many forensic scientists study exothermic reactions extensively. First they have to learn how various chemical reactions take place and how much heat is produced. As investigators, this knowledge better prepares them to find out what caused a fire or explosion and to determine the best way to extinguish it in the future.
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