Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Emery consists of impure (a) iron (b) corundum (c) carborundum (d) graphite

Step-by-Step Solution

Answer

Emery consists of impure (b) corundum.

Step 1: Understand the term 'Emery'

Emery is a naturally occurring rock that is rich in a mixture of minerals. It is particularly known for its abrasive properties which make it useful in grinding, polishing and sanding applications.

Step 2: Identify the Compositions

Emery mainly consists of corundum. Corundum is a crystalline form of aluminum oxide typically containing traces of iron, titanium, vanadium, and chromium which contribute to the hardness and abrasive characteristics of emery.

Step 3: Evaluate Other Options

Now, look at the other composed materials: (a) Iron is primarily a metal, not typically part of natural emery. (c) Carborundum, synthetically known as silicon carbide, is not a natural component of emery. (d) Graphite is used as a lubricant and is not abrasive like emery.

Step 4: Determine the Answer

From the above analysis, we deduce that emery consists primarily of corundum. Therefore, option (b) corundum is the correct choice.

Key Concepts

Corundum

Corundum is a fascinating mineral known for its hardness and durability. It is composed of aluminum oxide (Al₂O₃) and is typically colorless. However, it often has traces of other elements which can impart color variations:

• Iron
• Titanium
• Vanadium
• Chromium

These impurities can turn corundum into precious gemstones, such as rubies and sapphires. Corundum gets its name from the Tamil word "kurundam," which refers to this mineral group. Due to its durability, corundum is commonly used in industrial applications, especially as an abrasive.

Abrasive properties

The abrasive properties of corundum are what make it especially significant in various industrial uses. Abrasives are substances used to wear down, scour, or polish other surfaces through friction. The hardness of corundum is recorded as 9 on the Mohs scale, just one step below diamond. This exceptional hardness allows it to efficiently remove material from surfaces.
Common uses for abrasives include:

• Grinding wheels
• Sandpaper
• Polishing pastes
• Cutting tools

These tools rely on abrasives like corundum for their effectiveness. In particular, emery, with corundum as its key component, is utilized for its dependable abrasive action.

Minerals in emery

Emery is a unique rock primarily comprised of corundum, but it also includes several other minerals contributing to its abrasive qualities. These can usually be found in small percentages and together enhance emery's functionality, these include:

• Spinel
• Magnetite
• Hematite

These minerals, combined with corundum, create a versatile abrasive material. Emery's natural composition allows it to maintain a consistent texture and effectiveness in applications where precision and durability are required.

Grinding and polishing applications

Emery's application in grinding and polishing is vast, utilizing its natural abrasive properties. This helps in refining surfaces to a smooth finish and in shaping materials. Various industries capitalize on emery's effectiveness:

• Metalworking: To shape and smooth metal objects
• Jewelry making: For polishing gemstones and metal surfaces
• Woodworking: To smooth wooden surfaces for finishing
• Rubber and plastic industries: For fine-tuning the shape and texture of products

These applications highlight emery's versatility and importance in manufacturing and craft industries. The reliability of emery in consistently achieving desired textures and finishes makes it a staple material for these processes.