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aluminium reacts with oxygen to produce aluminium oxide, which is a white powder. The chemical formula for aluminium oxide is Al2O3. This compound is amphoteric, meaning that it can ionise in water and react with acids as well as bases. The ions form a crystal structure that is similar to corundum. It is also commonly used as abrasive in the manufacture of sandpaper and grinding wheels. It is also the main ingredient in some sodium vapor lamps.
Metallic aluminium does not normally react with water in air, but it can react to produce aluminium oxide if the surface is corroded. A layer of aluminium oxide, known as passivation, forms on the surface of aluminium to prevent it from reacting with oxygen in the air. The thickness of the passivation layer can be increased by a process called anodising. This is often done on aluminium alloys to increase their corrosion resistance, as well as for decorative effects.
When aluminium is exposed to acid, a layer of aluminium oxide forms on the surface of the metal. This protects the aluminium from the acid, but if it is damaged the acid will react with the exposed aluminium to produce toxic hydrogen gas.
Aluminium oxide can be produced by reacting elemental aluminium with diatomic oxygen (O2) in the air. This is a combination reaction because the reactants are different and both have a positive electrical charge. The molecular formula for the product is Al2O3, which has the same chemical properties as the pure elemental aluminium. The aluminium ions in the compound are closely packed together, but they have an uneven distribution of positive and negative charges so that the compound is electrically neutral.