Why can't metal catch fire in life?

Life experience tells us that metal won't burn, even if it is baked in a fire, it won't catch fire, otherwise you won't cook! In fact, metal will burn. Most metals release a lot of heat when burning, which is difficult to be extinguished.

Some examples of metal combustion

For example, thermite is used to weld train tracks together. The fuel in thermite is metallic aluminum. When thermite burns, aluminum atoms combine with oxygen atoms to form alumina, which releases a lot of light and heat in the combustion process.

Another example is holding a small fireworks stick and using aluminum, magnesium or iron as fuel. The flame of a pyrotechnic stick looks different from wood, because metal burns hotter, faster and more thoroughly than wood. In fact, most fireworks contain metal fuel. For another example, the old flash used in early photography only burned a small amount of magnesium in a glass ball, with a bang and a thick smoke. In addition, the solid rocket booster of the space shuttle uses aluminum as fuel. Some metals, such as sodium, burn easily, so we can't use sodium to make daily necessities.

Why don't metals in daily life catch fire?

However, you may think, why don't lit matches burn on aluminum foil? Similarly, putting a metal pot on the kitchen flame will not make the pot catch fire. In daily life, metal objects don't seem to burn. So can metals in daily life really burn? There are three main factors involved here.

Metal atoms can't combine well with oxygen and are not easy to vaporize. First of all, in solid metals, it is difficult for metal atoms to be close enough to oxygen atoms to react. In order to make the metal burn, each metal atom must be close enough to the oxygen atom to combine with it and have a chemical reaction. Large pieces of metal, such as spoons, cans, iron bumps, most of the metal atoms are buried in the depths, and there is no contact with oxygen molecules.

In addition, metals do not evaporate easily. If a large piece of wood or candle is burned, the fuel particles will evaporate quickly, that is, with a little heating, the atoms will evaporate into the air, so that they can better contact with oxygen atoms. In contrast, the atoms of solid metals are closely combined, which means that it is much more difficult to evaporate metals by heating.

In addition, organic materials such as wood or cloth contain a lot of oxygen atoms in their own molecules, while metal materials do not. This is one of the reasons why metal spoons are more difficult to burn than wooden spoons, even though they are all made of large pieces of material.

In view of the above facts, it is necessary to decompose metal atoms artificially in order to make them better and fully exposed to oxygen. In fact, it is artificially grinding metal into fine powder. When used as fuel in commercial products and industrial processes, metals usually exist in the form of powder. Even if we grind metal blocks into powder, the combustion effect is not good, and it is difficult for us to ignite a pile of metal powder in the air. The problem is that the air is mainly nitrogen and there is not much oxygen.

The best way is to mix oxygen atoms directly into metal powder. For example, solid compounds containing loosely bound oxygen atoms can be mixed into metal powder. In this way, oxygen atoms can stay stably next to metal atoms and be ready to react at any time. This method is the most effective way to make the metal burn fully. For example, thermite is a mixture of aluminum powder (fuel) and iron oxide (oxygen source).

The second reason for the high ignition temperature of metals is that everyday metal objects are not easy to catch fire, because metals usually have a high ignition temperature. Because the atoms in typical metals are closely related, even if oxygen atoms are beside them, more energy is needed to separate and release metal atoms. Candle flames, match flames, bonfires and kitchen stoves are not hot enough to ignite most metals, even if they are ideal powder forms. A stronger chemical reaction must be used to generate a higher temperature to ignite most metal powders. For example, the burning of magnesium bars can be used to ignite thermite.

Metals are good thermal conductors. The last reason why daily metal products are not easy to burn is that metals are often good thermal conductors. This means that if a point on a metal object starts to accumulate heat, the heat will quickly flow through the metal to the colder part. This makes it difficult for metals to gather enough heat in one place to reach the ignition temperature.

abstract

Because the atoms in most solid metals can't contact with oxygen atoms, because the metal has a high ignition temperature and is a good thermal conductor, it can't burn in daily environment. The ideal way to make the metal burn is to grind it into powder, add oxidant, stir it evenly, wrap it up so that the heat will not escape, and then ignite the metal with a high-temperature ignition device.