Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Pick out the incorrect statement among the following. (1) ferric alum is commonly known as Mohr's salt (2) cast iron cannot be welded (3) heating steel to a high temperature followed by quick quenching in water is called annealing (4) alnico is used for making utensils and automobile parts. (a) \(1,2,3\) (b) 1,3 (c) 2,4 (d) \(1,3,4\)

Step-by-Step Solution

Answer

Option (d) 1, 3, 4 are incorrect statements.

Step 1: Examine Statement 1

Statement (1) suggests that ferric alum is commonly known as Mohr's salt. Mohr's salt, however, is actually ferrous ammonium sulfate, not ferric alum. Thus, Statement (1) is incorrect.

Step 2: Examine Statement 2

Statement (2) states that cast iron cannot be welded. Cast iron can be welded using special techniques and equipment, although it is more challenging than welding other metals. Therefore, Statement (2) is questionable but not strictly incorrect. It can be considered correct with a certain context.

Step 3: Examine Statement 3

Statement (3) suggests that heating steel to a high temperature followed by quick quenching in water is called annealing. This is incorrect, as the process described is actually called quenching or hardening, not annealing. Annealing involves heating and then slowly cooling the metal to soften it.

Step 4: Examine Statement 4

Statement (4) mentions that alnico is used for making utensils and automobile parts. Alnico is an acronym referring to an alloy made of aluminum, nickel, and cobalt, primarily used for making magnets, not utensils or automobile parts. Hence, Statement (4) is incorrect.

Step 5: Determine the Incorrect Statements

Upon evaluating all statements, Statements (1), (3), and (4) were found to be incorrect.

Key Concepts

Ferric Alum

Ferric alum is a chemical compound that is largely used in water purification and in the dyeing industry. It is an aluminum sulfate-hydrated compound that contains iron. Unlike Mohr’s salt, which is ferrous ammonium sulfate and used as a standard laboratory reagent, ferric alum should not be confused as they serve different purposes. Ferric alum is often used in the textile industry as a mordant, which helps to fix dyes onto fabrics.
It is also used in the tanning industry for its astringent properties.

Mohr's Salt

Mohr's salt, known scientifically as ferrous ammonium sulfate ext {(NH}_4 ext{)_2Fe(SO}_4 ext{)_2 ext{.6H}_2 ext{O)}, is a double salt used as a reagent in analytical chemistry. Unlike ferric alum, it does not contain aluminum. Instead, it is a prominent part of volumetric analysis due to its stability and resistance to oxidation, making it ideal for titration processes.
Mohr’s salt is typically used when preparing standard solutions due to its high purity and reliability under normal laboratory conditions.

Annealing Process

Annealing is a metallurgical process primarily used to alter the physical and chemical properties of a material to reduce its hardness and increase its ductility. The goal of annealing is to make the metal more workable. This process involves heating the metal to a specific temperature before allowing it to cool slowly, usually in air.
By slowing down the cooling process, the internal structures of the metal have time to relax, resulting in a softer, more ductile metal. This is the opposite of quenching, which is designed to harden metal.

Quenching Process

Quenching is a rapid cooling process used in metallurgy to increase the hardness and strength of a metal. In this process, the metal is initially heated to a high temperature and then quickly cooled in water, oil, or air. This rapid cooling process "locks" the crystal structure of the metal, often resulting in increased hardness and brittleness.
Quenching is commonly used in steel production to achieve the desired mechanical properties; however, care must be taken to avoid defects such as warping or cracking due to the intense temperature change.

Alnico Usage

Alnico is an alloy composed mainly of aluminum, nickel, and cobalt. It is primarily used for manufacturing permanent magnets due to its capability of retaining a strong magnetic field. Unlike common misconceptions, alnico is not used for making utensils or automobile parts.
Its properties make it ideal for use in loudspeakers, electric guitars, and various electronic and industrial applications requiring strong yet stable magnets.

Metallurgical Processes

Metallurgical processes encompass a variety of methods used in the extraction and processing of metals. These processes include smelting, refining, and forming metal alloys. Annealing and quenching are just two examples of metallurgical techniques used to impart desired properties to metals.
Each process aims to improve the performance, durability, and appearance of metals according to their intended applications. Understanding these processes is vital for industries such as construction, manufacturing, and material science, where metal integrity and functionality are crucial.