Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

Silver containing lead as an impurity is removed by (a) distillation (b) poling (c) levigation (d) cupellation

Step-by-Step Solution

Answer

(d) cupellation

Step 1: Understanding the Methods

First, we need to understand what each method entails: - **Distillation:** This is a process of separating components based on differences in their boiling points by evaporating and then condensing the vapor. - **Poling:** This is a method where metal is refined by stirring it with green wood poles. Useful for removing oxides. - **Levigation:** This involves washing ore over a sloped surface so that heavy metals remain as lighter ones are washed away. - **Cupellation:** This is a refining process where lead-containing silver is heated in a cupel. Lead turns into lead oxide and is absorbed by the porous cupel or removed by airflow.

Step 2: Identifying the Correct Method

Given the definitions: - Silver containing lead can be best purified using a method that removes or absorbs lead. - Cupellation specifically involves refining precious metals such as silver and gold by removing lead. - During cupellation, the lead is turned into lead oxide at high temperatures and is absorbed into the cupel, leaving behind pure silver.

Step 3: Conclusion

Cupellation is the only method listed that is known specifically for removing lead from silver by turning lead into lead oxide, which is then absorbed in a cupel. Therefore, the method to remove lead impurities from silver is cupellation.

Key Concepts

Cupellation Method

The cupellation method is a fascinating technique used primarily for refining silver that is contaminated with lead. This process is particularly effective due to its unique ability to separate lead from silver, leaving behind a more refined form of the precious metal. During cupellation, the silver-lead alloy is placed in a special receptacle known as a cupel. This cupel is made from materials that can withstand intense heat and have the ability to absorb non-metallic elements. The alloy is then exposed to high temperatures, causing the lead to react with oxygen in the air and form lead oxide.
The magic of this process lies in the fact that lead oxide possesses different properties compared to pure lead:\[\text{Lead} \quad + \quad \text{Oxygen} \quad \rightarrow \quad \text{Lead Oxide (absorbed)}\]
Once formed, the lead oxide is either absorbed by the cupel or removed through airflow. This leaves behind purified silver. It's crucial in refining processes where the goal is to achieve high purity levels of precious metals. By efficiently removing lead, cupellation highlights the ability of chemical reactions to aid in metallurgy, ensuring more desirable properties in the resulting metal.

Refining Precious Metals

Refining precious metals involves various methods to increase the purity of metals like gold, silver, and platinum. Metals are often found mixed with impurities or other less valuable elements, which necessitates a purifying process. Depending on the specific metal and type of impurities, several methods can be employed.
Here are a few common techniques:

• **Cupellation:** Primarily used for silver with lead impurities as explained previously. It is particularly important when working with silver to ensure the removal of lead.
• **Electrolytic Refining:** This method uses an electrical current to drive the purification process. It is widely used for refining copper and gold.
• **Parting:** Known for its application in separating gold and silver. This process uses strong acids to dissolve one metal and not the other.

Each method chosen depends on the specific impurities involved and the required purity level of the final product. These techniques are not only pivotal in achieving high-purity metals but also in enhancing their industrial applicability.

Chemical Processes in Metallurgy

Chemical processes in metallurgy are integral to transforming raw ore into usable metals. These processes involve a series of reactions targeting the removal or transformation of unwanted elements. The efficacy of these reactions plays a vital role in refining metal properties and removing impurities.
In metallurgy, chemical processes can involve:

• **Oxidation-Reduction Reactions:** Essential for changing the oxidation states of metals, often used in processes like smelting where metal oxides are reduced to pure metals.
• **Acid and Alkaline Treatments:** These strategies help dissolve unwanted minerals or produce metal salts, which can be further processed into pure metals.
• **Leaching:** A chemical separation process that extracts valuable metals from ores, especially when dealing with low-grade ores.

Understanding these processes is crucial as they underpin the techniques applied in refining and forming metals for various applications. From a basic ore to a metal ready for the marketplace, chemistry plays a transformative role in metallurgy, empowering us with materials capable of building the modern world.