When a substance rapidly reacts with oxygen, energy is released in the form of heat and/or light. This type of exothermic reaction is known as combustion (commonly called burning). There are three main requirements for combustion: fuel, oxygen, and heat. For example, if hydrogen is exposed to sufficient heat to ignite it in the presence of oxygen, it will burn. The hydrogen atoms are the fuel, which react with oxygen to produce water and heat. The chemical equation for hydrogen combustion is written as: 2H₂ + O₂ --> 2H₂O + heat. Hydrogen would not burn if oxygen were not available, nor would the reaction be triggered without heat. Lacking any one of the three necessary ingredients, combustion would not be possible.

Substances that ignite at conditions and temperatures common to everyday life are considered flammable. However, certain substances that are not labeled as flammable are still combustible under the right circumstances. For example, lycopodium powder (a yellow powder made from the spores of a moss) is not particularly flammable under normal circumstances: if you were to try to light a pile of lycopodium powder, it would not burn. However, lycopodium powder becomes extremely combustible when it is suspended in air. This is because there are other factors—temperature, the concentration of the reactants, and the surface area of the fuel, for instance—that affect combustion.

In general, temperature affects the kinetic energy of the reactants: the higher the temperature, the more energy the atoms have and the faster they move. To have a chemical reaction, the reactants need to collide with enough energy to trigger the reaction. In addition, the concentration (amount) of reactants affects the frequency of collisions between the atoms: the more collisions, the faster the rate of reaction. Similarly, an increase in surface area of a reactant also increases the frequency of collisions.

Many fine solids, like lycopodium powder, are dangerous in their dust form. As a dust, the surface area of a material is high. In addition, when dust is airborne, the particles are surrounded by oxygen. As a result, combustion can happen at a very rapid rate. The rapid combustion can lead to an explosion if the resulting heat and gas cannot dissipate quickly enough. Dust explosions are a serious hazard in many industries, and extreme care must be taken in factories to control dust and to prevent ignition. Dust from wheat, corn, wood, plastic, and even metals can produce dangerous combustion reactions.

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