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Dr. Christopher S. Baird

Why don't metals burn?

Category: Chemistry      Published: February 18, 2018

fireworks
Fireworks are an example of burning metals. Public Domain Image, source: OSHA.

Metals do burn. In fact, most metals release a lot of heat when they burn and are hard to put out. For example, thermite is used to weld train rails together. The fuel in thermite is the metal aluminum. When thermite burns, the aluminum atoms bond with oxygen atoms to form aluminum oxide, releasing a lot of heat and light in the process. As another example, hand-held sparklers use aluminum, magnesium, or iron as the fuel. The flame of a sparkler looks different from the flame of a wood fire because metal tends to burn hotter, quicker, and more completely than wood. This is what gives a lit sparkler its distinctive sparkling flame. In fact, most fireworks contain metal fuels. As another example, old flash tubes used in photography were nothing more than burning bits of magnesium in a glass bulb. Also, the space shuttle's solid rocket boosters used aluminum as the fuel. Some metals, such as sodium, burn so well that we don't make everyday objects out of them. Any boy scout who has started a fire using steel wool can attest to the fact that metals burn.

Still, you may wonder why holding up a lit match to aluminum foil does not make it burn. Similarly, placing a metal pan on a kitchen flame does not make the pan burst into flames. In everyday situations, metal objects don't seem to burn so much. How can this be possible if metals actually do burn? There are three main factors involved.

First, if you have a solid chunk of metal, it is hard to get oxygen atoms close enough to the majority of the metal atoms to react. In order to burn the metal, each metal atom has to get close enough to an oxygen atom to bond to it. For large chunks of metal; like spoons, pots, and chairs; most of the atoms are simply too deeply buried to have any access to oxygen molecules. Furthermore, metals don't vaporize easily. When you burn a chunk of wood or a wax candle, the fuel particles readily vaporize, meaning that with just a little heat, they shoot out into the air where they have better access to oxygen atoms. In contrast, solid metals tend to have their atoms very tightly bound together, meaning that it is much harder to use heat to vaporize the metal. Also, organic materials like wood or cloth contain a lot of their own oxygen, whereas raw metals don't. This is one reason why it is much harder to burn a metal spoon than a wooden spoon, even though they both consist of large chunks of material.

With this fact in mind, all we have to do is manually break apart the metal atoms in order to get them to burn better. In practice, this means grinding the metal down to a fine powder. When used as a fuel in commercial products and industrial processes, metals usually come in the form of a powder. Although, even if you have ground a metal block down to a powder, it still won't burn as efficiently as it could if you just use the oxygen in the ambient air. The problem is that air does not actually contain that much oxygen. Air is mostly nitrogen. The best approach is to mix oxygen directly into the powder. Raw oxygen won't work so well because it is a gas at room temperature and will float away. Instead, solid compounds containing loosely bound oxygen atoms can be mixed into the metal powder. In this way, the oxygen atoms can stably sit right next to the metal atoms, ready to react. This approach is the most efficient way to get metals to burn well. For example, thermite is just aluminum powder (the fuel) mixed in with iron oxide (the oxygen source).

The second reason that everyday metal objects don't burn so well is that metals generally have a higher ignition temperature. Because the atoms in a typical metal are so tightly bound to each other, it takes more energy to break them apart and free them up, even if the oxygen atoms are sitting right next to them. Candle flames, match flames, campfires, and kitchen stove flames simply don't get hot enough to ignite most metals, even if the metal is in the ideal powder form. Chemical reactions that produce higher temperatures must be used to ignite most metals. For example, the combustion of magnesium strips can be used to ignite thermite.

The last reason that everyday metal objects don't burn so well is that metals tend to be excellent thermal conductors. This means that if a spot on a metal object starts to build up some heat, the heat very quickly flows through the metal to cooler parts of the object. This makes it hard to build up enough heat in one spot to reach the ignition temperature. Even if you have a flame torch running at a high enough temperature, it is difficult to use the torch to ignite a chunk of metal because the heat keeps flowing away through the metal.

In summary, because most atoms in a solid chunk of metal don't have access to oxygen atoms, because metals have a high ignition temperature, and because metals are good thermal conductors, they don't burn very well in everyday situations. The ideal way to get a metal to burn is to grind it into a powder, mix in an oxidizer, contain it so heat can't escape, and then apply a high temperature ignition device.

Topics: atom, atoms, burn, combustion, fire, metal, metals

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