Aluminium oxide

Aluminium oxide

Samples of ruby, a red variant of corundum.
Names
IUPAC name Aluminium(III) oxide
Systematic IUPAC name Aluminium oxide
Other names Aloxide Aloxite Alumina Alundum Corundum Ruby Saphire
Identifiers
Jmol-3D images	Image
SMILES [Al+3].[Al+3].[O-2].[O-2].[O-2]
Properties
Chemical formula	Al2O3
Molar mass	101.96 g/mol
Appearance	White solid
Odor	Odorless
Density	3.95–4.1 g/cm3
Melting point	2,072 °C (3,762 °F; 2,345 K)
Boiling point	2,977 °C (5,391 °F; 3,250 K)
Solubility in water	Insoluble
Solubility	Reacts with halogenic acids and alkali Insoluble in organic solvents
Vapor pressure	~0 mmHg
Thermochemistry
Std molar entropy (So298)	50.92 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	−1675.7 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich
Flash point	Non-flammable
Related compounds
Related compounds	Boron trioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Aluminium oxide or alumina is the chemical compound of aluminium and oxygen, with chemical formula Al₂O₃. It's the most common and stable form of the aluminium oxides. Aluminium oxide occurs naturally in it's crystalline alpha phase as mineral corundum.

Properties

Chemical

Aluminium oxide will react with sodium hydroxide to form sodium aluminate.

Physical

Aluminium oxide is a white solid compound, insoluble in water and solvent, but will dissolve in non-oxidizing acids. It is odorless and it's very hard (9 Mohs scale). It's an electrical insulator.

Availability

Aluminium oxide is often present in sandpapers, you can also buy it from mineral collectors as corundum or gemstones like ruby or sapphire. Certain smartphones have colorless transparent protective sheet of synthetic corundum.

Heating elements also contain alumina powder which acts as an insulator for the resistance wiring.

Preparation

Aluminium oxide is a product of thermite reaction:

M₂O₃ + 2 Al → Al₂O₃ + 2 M

This reaction also gives various aluminates, and separating the alumina from the slag is complicated and may not worth it.

A much better and simpler method involves the dehydration of aluminium hydroxide, which can be obtained by precipitating an aluminium salt with sodium or potassium hydroxide.

Al(NO₃)₃ + 3 NaOH → Al(OH)₃ + 3 NaNO₃

Another route involves adding an acid to sodium aluminate, forming sodium salt of said acid and aluminium oxide/hydroxide. Excess NaOH can be added after the addition of the acid to prevent the formation of aluminium salts. Sodium aluminate itself is obtained from the reaction of sodium hydroxide and aluminium metal.

NaOH + Al → NaAlO₂ + H₂O + H₂

NaAlO₂ + H⁺ + NaOH → Na⁺ + Al(OH)₃

Aluminium oxide can be obtained by calcinating aluminium hydroxide between 500-850 ºC:

2 Al(OH)₃ → Al₂O₃ + 3 H₂O

Projects

• Make aluminium salts

Handling

Safety

No special handling is necessary, though it's recommonded to avoid inhaling in in powdered form. Aluminium oxide isn't flamable or explosive.

Storage

Storage in closed bottles is adequate.

Disposal

Aluminium oxide could be safely dumped with normal trash or spilled out in a soil.

References

Relevant Sciencemadness threads

• Making Relatively Pure Alumina