  Aluminum is a shiny silvery metal, but you rarely see it in its natural state. Aluminum reacts easily with the oxygen in the air; as soon as the metal is exposed to air, its outer surface transforms to aluminum oxide, providing the familiar dull grey outer coating on the metal. This thin outer coating then serves to protect the aluminum from any further reactions.Aluminum can also be anodized, which thickens and hardens the aluminum oxide coating, making it shiny like pure aluminum. The reaction of aluminum with oxygen is an example of an oxidation reaction (which just means combining chemically with oxygen). A similar oxidation reaction is the combining of iron with oxygen to form iron oxide; we call this process by a more familiar name: rusting. Both of these reactions are exothermic, but at room temperature the heat produced is small and is quickly dissipated into the surroundings. It is a different story when aluminum is heated to a high temperature first, before exposing it to oxygen. When that happens, the reaction with oxygen happens very fast, and releases a lot of heat ... so much heat, in fact, that you can stop applying heat to the aluminum; the heat from the reaction will be enough to keep the process of rapid oxidation going. Not only will the reaction be self-sustaining, but you don't even have to rely on the oxygen in the air to keep it going. You can instead add a compound containing oxygen to the process, and the reaction will occur by pulling oxygen from this material. This makes for some very useful applications, as you will see.  
Let's look at thermite welding, which is often used to weld steel railway tracks together. Railway workers insert thermite (a mixture of aluminum powder and iron oxide) between the two rails to be welded together.A ribbon of magnesium is ignited to provide the high temperature needed to start the reaction. Then the aluminum combines with the oxygen from the iron oxide, releasing sufficient heat to maintain the reaction. The left-over metallic iron (from the iron oxide) melts in the high heat, and fills the gap between the rails!   Oxidizing aluminum is also used in solid core rockets for propulsion. A familiar example is the space shuttle, which uses two strap-on solid fuel rockets as well as a large liquid fuel tank. The solid core boosters are lined with a solid mixture of aluminum powder and ammonium perchlorate. This latter substance is the oxidizer ... it provides the oxygen for the reaction. As the mixture reacts, aluminum converts to aluminum oxide, which is vaporized by the high temperature; the escaping gases provide thrust to the rocket. Once the solid fuel (aluminum and ammonium perchlorate) has been ignited, there is no stopping the reaction until the fuel is spent; solid booster rockets can't be stopped and restarted like liquid fueled ones. The shuttle also has a liquid fuel tank which holds both fuel and oxidizer; hydrogen (stored as a liquid) combines with oxygen (stored as a liquid) to produce water vapor and heat; high temperature water vapor escaping from the rocket adds to the thrust. Ammonium perchlorate is a chemical compound with the formula NH4ClO4. It is a powerful oxidizer. It generates toxic gas and extremely high temperatures as it decomposes. It is used in solid rocket propellants, as described above, as well as in some fireworks.  In a solid fuel rocket like those strapped to the space shuttle, oxygen from ammonium perchlorate combines with aluminum to produce All the fuel is burned within a few minutes. This equation describes the reaction:  ... plus enormous quantities of heat With the space shuttle, in addition to the two solid-fuel rocket boosters (which provide only about 70% of the total thrust at lift-off), there is also a main engine on the orbiter whose fuel (hydrogen) combines with an oxidizer (oxygen), both of which are stored in the main orange external tank. The equation for this much simpler reaction is the familiar one that creates H2O:  "Dr. Joe & What You Didn't Know" by Dr. Joe Schwarcz |