An exothermic reaction is a chemical reaction that produces heat. The term exothermic is composed of the root exo, which is Greek for outside, and thermic, which means heat. Therefore, exothermic defines heat going outside. Exothermic reactions are important to forensic sciences and particularly to fire and explosion investigation. When a chemical reaction requires heat (rather than producing it) and results in cooling down the surroundings, it is conversely called endothermic.
Exothermic reactions occur in many phenomena and applications of every-day life. The speed at which they occur can range from extremely slow to extremely fast. For example, an exothermic reaction occurs when a piece of steel rusts. Rust is iron oxide (Fe2O3), which is produced by the reaction of iron (Fe) with oxygen (O2). This reaction releases heat and is therefore, exothermic. However, it takes place at such a slow pace that it is impossible to observe a difference of temperature on the piece of steel. Fire is an exothermic reaction that occurs much faster. When a fuel is burning, it releases heat, which can easily be felt. An explosion is also an exothermic reaction. Even if the end result of an explosion is the high pressure generated that pushes everything away from its seat, heat is first produced in great magnitude.
All combustion reactions are exothermic. In a fire investigation , it is important to know what kind of chemical reaction occurred and caused a fire. In order to do that, it is necessary to know the substances involved and how they react with one another. Some reactions also require a threshold amount of energy given to them before they can release (more) energy. This is called the energy of activation. For example, it is necessary to light wood on fire with a lighter or matches before it will burn and liberate heat.
Finally, it is possible to measure the amount of heat or energy liberated by a given chemical reaction. In order to do this, an instrument called a calorimeter is used. The calorimeter is a container placed in a well-insulated water bath. A known amount of the substance or substances that will react is placed in the container and the reaction takes place. Ideally, all the heat generated by the reaction is transferred to the water, whose temperature increases. By monitoring the temperature of the water, it is possible to accurately measure the amount of heat liberated.
see also Chemical equations; Explosives.