ANS:C - aluminium oxide

Aluminium oxide, also known as alumina, is a chemical compound composed of aluminium and oxygen with the chemical formula Al2O3. It occurs naturally as the mineral corundum, and it's widely used in various industries due to its exceptional properties. Here's an explanation of some key aspects of aluminium oxide:

1. Hardness: Aluminium oxide is renowned for its hardness, making it one of the hardest materials known. It ranks very high on the Mohs scale of mineral hardness, just below diamond. This property makes it valuable for abrasive applications such as grinding wheels, sandpaper, and cutting tools.
2. Abrasive Properties: Due to its hardness and abrasive nature, aluminium oxide is extensively used as an abrasive material. It's commonly employed in abrasive media for sandblasting, as well as in grinding and polishing applications where a hard and durable abrasive is required.
3. Thermal Stability: Aluminium oxide exhibits excellent thermal stability, retaining its strength and hardness at high temperatures. This property makes it suitable for use in various high-temperature applications, including refractory materials, furnace linings, and thermal insulators.
4. Chemical Inertness: Aluminium oxide is chemically inert, meaning it does not react easily with other substances under normal conditions. This property contributes to its corrosion resistance and makes it useful in applications where exposure to corrosive environments is a concern.
5. Electrical Insulation: In addition to its mechanical properties, aluminium oxide also possesses good electrical insulation properties. It's commonly used as an insulating material in electrical components and insulators.

Overall, aluminium oxide is a versatile material with a wide range of applications across industries such as metallurgy, ceramics, abrasives, electronics, and more, thanks to its combination of hardness, thermal stability, chemical inertness, and electrical insulation properties.