Aluminium oxide is not aluminium!

After the first surprise of such a shortcut, it is fair to realise that the concept of oxide is not intuitive. Oxide, oxidation, anti oxidation. The first thing that might come to mind is that drinking green tea is good for your health and to keep you young because it is full of antioxidants. We are a bit far from aluminium oxide.

Aluminium and aluminium oxide are very different even thought they are related. Let’s first see why it is so important to make the difference, then why assuming they are the same is –kind of- understandable, not only because of the name, but also because if you get one you can get the other.

You know what is aluminium. It is a grey metal, with a lighter weight than other metals. Its chemical symbol, or abbreviation, is “Al”. As a metal, it conducts electricity and heat. It is quite ductile, meaning that you might be able to deform an aluminium bar with your own hands, while the same bar made of iron is more rigid and difficult to deform. It is very common, you probably have some in your kitchen in the form of aluminium foil.

So what is aluminium oxide? It is a ceramic. It is white, it does not conduct electricity, it is very hard. For the same volume, it is heavier than aluminium.

Then why is it called aluminium oxide? “Oxide” derives from “oxygen”. The chemical formula is Al2O3, meaning that there are aluminium atoms, Al, surrounded by oxygen atoms O. It is very abundant on earth but you might not have it raw around you: rubies and sapphires are forms of aluminium oxides. But many other objects do contain some aluminium oxide.

Indeed, any aluminium object (metallic object thus), is covered by a very thin layer of aluminium oxide. This layer forms spontaneously in contact with the oxygen from the air. Similar types of layer also form on other metals, and protect them from detrimental degradation. It is called a passivation layer. It is so thin that you cannot really tell the difference from a freshly polished aluminium surface, and it does not affect much the properties of the underlying metal.

We mentioned earlier, aluminium oxide is a ceramic. It is also one major component of earth, clay, porcelain. Natural porcelain for example contains roughly 45 % of aluminium oxide, which is partly responsible for its white color. Between ceramists, aluminium oxide has a second little name, “alumina”.

It is important to make the difference between the aluminium-metal and the aluminium-oxide-ceramic, but if aluminium-metal comes with aluminium-oxide-ceramic, it is also possible to get aluminium-metal from aluminium-oxide-ceramic. This is actually how the metal is extracted from the soil. Earth, as we have seen, contains high amount of aluminium oxide. To be more correct, it contains a hydrated form of aluminium oxide, called bauxite. Depending on the degree of hydration its chemical symbol can be Al(OH)3 or AlO(OH). You notice the –H compared to the aluminium oxide Al2O3. The –H comes form water H2O.

The process of extraction of aluminium from hydrated aluminium oxide is called the Bayer process and is still the most widely used industrial method since its creation at the end of the 19th century.

Similar to aluminium, iron and iron oxide are two different materials, as well as magnesium and magnesium oxide, titanium and titanium oxide, etc. Atoms are tiny but big enough to make a drastic change in the material!

I invite you to look at this website to know more about the production of aluminium: http://www.aluminiumleader.com/production/aluminum_production/

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