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In chemistry, aluminium oxide is acidic because it has an atom-for-atom reaction with water to produce aluminium hydroxide. This is because water has a lower electronegativity than aluminium, and so will bond more easily with it. The hydroxide ions are then able to form a stronger covalent bond with the aluminium than they would have been able to do without the water.
This is a very important property of aluminium oxide, which makes it useful as an abrasive and for many other uses. It is also a common ingredient in sunscreens and cosmetics such as nail polish and blush, and it is used in the manufacture of glass. In aluminosilicate glass it is usually present in quantities of around 5-10%.
Aluminium oxide has the chemical formula Al2O3. In its natural crystalline form, it is known as corundum or alumina. It is a very hard mineral, rating 9 on the Mohs scale of mineral hardness, and it is used as an alternative to industrial diamond in the manufacture of sandpaper and cutting tools. It is also a major component of the cue tip chalk used in billiards, and it is found in some CD/DVD polishing and scratch-repair kits.
Most metal oxides are basic, but aluminium oxide is an amphoteric oxide, showing both acidic and basic properties. It reacts with acids such as hydrogen fluoride to give aluminium fluoride, but it also reacts with bases such as sodium hydroxide to produce salt and water. This is because the difference in electronegativity between aluminium and oxygen is much smaller than it is for most metals, and so aluminium can form covalent bonds with oxygen as well as with itself.