Q. Show the properties of Aluminium Oxide ?

Aluminium oxide, Al₂0₃ is a highly ionic compound of aluminium. It is also known as alumina. As you have studied earlier, it occurs in nature not only as bauxite and colourless corundum, but also as coloured gem stones like ruby, emerald, sapphire and topaz, etc. Colours of these gem stones are due to the presence of transition metal ions. Anhydrous aluminium oxide can exist in two forms both of which are white. These are a-Al₂0₃ and 7-AI₂O₃. The 7-AI₂O₃ is formed by dehydrating AI (OH) ₃ below 750 K:

2AI (OH)₃--------------------------------------> 7-Al₂O₃ + 3H₂0

The 7-Al₂0₃ is quite reactive. It dissolves readily in both acids and bases. If the 7-AI₂O₃ is heated to 1500 K, it changes to the a-AI₂O₃, which is called corundum. Corundum has a very high melting point (about 2328 K). It is very hard and inert,-especially towards acids.