Animated Solution for Chemistry - Metallurgy: Match List-I with List-II. \begin{array}{ll} \text{\textbf{List-I}} & \text{\textbf{List-II}} \\ \text{A. Mercury} & \text{1. Vapour phase refining} \\ \text{B. Copper} & \text{2. Distillation refining} \\ \text{C. Silicon} & \text{3. Electrolytic refining} \\ \text{D. Nickel} & \text{4. Zone refining} \end{array} Choose the most appropriate answer from the options given below.

The final stage of metallurgy is refining, a crucial step where crude metals are stripped of their remaining impurities to achieve the desired level of purity. The choice of refining method is not random; it is meticulously selected based on the unique physical and chemical properties of both the metal and its impurities. Let's break down the refining methods for Mercury, Copper, Silicon, and Nickel.

The Volatile Liquid

Mercury is unique among metals because it exists as a liquid at room temperature. This physical state implies that the intermolecular forces holding mercury atoms together are relatively weak, resulting in a very low boiling point.

Because of this high volatility, distillation is the perfect refining method for mercury. When crude mercury is heated, it readily vaporizes, leaving behind non-volatile impurities like lead or zinc in the distillation flask. The pure mercury vapor is then channeled into a condenser, where it cools and returns to its liquid state, now highly purified.

The Conductor's Choice

Copper is the backbone of the electrical industry. However, even trace amounts of impurities can drastically reduce its electrical conductivity. To achieve the exceptional purity required for wiring, we employ electrolytic refining.

In this process, a massive block of impure copper is made the anode (positive electrode), while a thin sheet of pure copper serves as the cathode (negative electrode). Both are immersed in an electrolyte solution of copper sulfate acidified with sulfuric acid. When an electric current is passed through the setup, copper atoms at the anode lose electrons and dissolve into the solution as $C{u}^{2+}$ ions. These ions migrate to the cathode, gain electrons, and deposit as pure copper. Impurities either dissolve in the solution or fall to the bottom as "anode mud," which often contains valuable precious metals like silver and gold.

The Heart of Electronics

Silicon is the foundational material for modern electronics and solar cells. For a semiconductor to function correctly, it requires an astonishing purity level—often referred to as "nine nines" ($99.9999999\%$). Standard chemical methods cannot achieve this, so we turn to zone refining.

Zone refining exploits a specific thermodynamic principle: impurities are generally more soluble in the molten state of a metal than in its solid state. A circular mobile heater is slowly passed along a rod of impure silicon. As the heater moves, it creates a narrow molten zone. The impurities prefer to stay in the liquid phase, so they are continuously swept along with the molten zone toward one end of the rod. After several passes, the impurities are concentrated at one end, which is then simply cut off and discarded, leaving behind an ultra-pure silicon crystal.

The Vapour Phase Elegance

Nickel is refined using a highly specific chemical technique known as Mond's process, which is a prime example of vapour phase refining. This method requires the metal to form a volatile compound that can later be easily decomposed.

In Mond's process, crude nickel is heated in a stream of carbon monoxide gas at around $330-350\text{ K}$. The nickel reacts to form a highly volatile coordination complex called nickel tetracarbonyl ($Ni(CO{)}_4$), while the solid impurities are left behind.

This gaseous complex is then transferred to a different chamber and heated to a higher temperature of $450-470\text{ K}$. At this heat, the complex becomes unstable and decomposes, depositing pure nickel metal and releasing the carbon monoxide gas, which is recycled back into the process.

By understanding the underlying principles, the matching becomes straightforward: Mercury relies on its low boiling point for Distillation refining ($A\to 2$). Copper requires high purity for conductivity, achieved via Electrolytic refining ($B\to 3$). Silicon needs semiconductor-grade purity, obtained through Zone refining ($C\to 4$). Nickel forms a volatile complex, making it perfect for Vapour phase refining ($D\to 1$).

This logical deduction leads us directly to the correct sequence: $A\to 2$, $B\to 3$, $C\to 4$, $D\to 1$.