Characteristics of aluminum and its combustion
Making it burn
Aluminum is a silvery-white metal that swiftly oxidizes in air and becomes covered with an oxide film. This reaction also occurs when the metal reacts with concentrated acids.
General characteristics of aluminum and its physical properties
Aluminum is the 13th element of the main group (IIIa, or boron group) of the periodic table. Aluminum has strong metallic properties, and its atomic weight is 26.98; the metal does not have stable isotopes in nature and exists in a single form. Aluminum has 3 valence electrons, and a great majority of aluminum compounds have an oxidation state of +3. Like all active metals, aluminum is a strong reducer, as it has a low electron affinity and a large atomic radius.
Aluminum is a light, soft, corrosion-resistant, highly-durable metal. Not every substance can boast such characteristics. Aluminum’s main physical properties are:
- melting point – 660 °C;
- face-centered cubic crystal structure;
- boiling point – 2470 °C;
- density – 2.7 g/cm3;
- type of bond – metallic;
- as aluminum is highly ductile and pliable, it is used to make durable, light, thin foil. It is also rolled into wire.
Aluminum’s reductive ability and chemical properties
Aluminum’s reductive properties can be observed in the element’s reactions with oxides of less active metals. Here’s an example of one such reaction equation:
Cr₂O₃ + 2Al = Al₂O₃ + 2Cr
In industry, aluminum’s reductive qualities allow for its use in obtaining other metals. When pure, aluminum is a strong reducer with high chemical activity. To increase aluminum’s activity, its oxide film must be removed. The chemical properties of the simple substance are determined by its ability to react with alkalis, acids, sulfur, and halogens. The metal does not react with water in ordinary conditions. At the same time, the only halogen that aluminum reacts with when unheated is iodine. Other reactions require the application of heat. Here you can learn more about how aluminum reacts with hydrogen and other substances.
Combustion of aluminum — reaction description
Pure aluminum particles do not combust in air or water vapor at temperatures below 1727 °C. To ignite aluminum in air, burning magnesium particles are placed on the surface of a heating element, and aluminum particles are placed on needle points above them.
The aluminum particles ignite in the vapor phase, and the intensity of the glow that appears around the particles slowly increases. Combustion is characterized by the presence of a glowing zone, which does not change size until the metal has burnt almost completely. In this zone, small drops of the oxide form and collide with one another. The particles remaining after combustion are shells with no metal inside. Here is the formula of the reaction of the combustion of aluminum in oxygen:
4Al + 3O₂ = 2Al₂O₃
Combustion in water vapor: the ignition of aluminum in water vapor is heterogeneous. The hydrogen released in the reaction helps destroy the oxide film, and the liquid aluminum oxide scatters in the form of drops. This oxide film forms and is destroyed repeatedly, as a significant percentage of the metal burns on the surface of the particles. Aluminum combusts five times more quickly in water vapor than in air.
Discovery of aluminum combustion
The combustion of aluminum powder in a mixture with oxygen gas was first applied in 1930 by chemists Becker and Strong in an oxygen-aluminum blowtorch they invented. The scientists used fine aluminum powder as fuel. To stabilize combustion, a device formed and constantly supplied a homogenous suspension of aluminum powder in oxygen. The mixture was lit with a Bunsen burner. It burned with a blinding white flame, releasing a large quantity of aluminum oxide smoke. These particles were so small that the smoke did not settle for 24 hours. Becker and Strong established that the combustion products contained around 2% free aluminum. By testing the effect of the blowtorch’s flame on various materials, the scientists approximately determined the temperature of the flame. Molybdenum melted, but a tungsten thread with a thickness of 1 mm did not. The scientists thus established that the combustion temperature of aluminum in the mixture with oxygen was between 2535 °C (the melting point of molybdenum) and 3400 °C (the melting point of tungsten). To observe the combustion reaction of aluminum, and admire the impressive sparks that appear as a result, you can conduct the following experiment: sprinkle aluminum powder into the flame of a burning spirit burner, using a porcelain spoon or spatula to add it in very small doses. You can also perform this same experiment with zirconium, titanium, or magnesium powder. When the metal powders burn, the oxides Al₂O₃, ZrO₂, TiO₂, and MgO form. You shouldn’t use extremely fine powders of these metals, as they may explode in the flame.