Question

In the electrolysis of alumina to obtain aluminium metal, cryolite is added mainly to:

• A. Lower the melting point of alumina
• B. Dissolve the alumina in molten cryolite
• C. Remove impurities from alumina
• D. Increase electrical conductivity

Answer 1

The Correct Option is A

In the process of extracting aluminum through the electrolysis of alumina, cryolite (Na₃AlF₆) plays a crucial role. Here is why cryolite is added:

1. Melting Point Reduction: Alumina (Al₂O₃) has a very high melting point of approximately 2072°C. Cryolite serves to significantly lower this melting point to around 900°C to 1000°C, making the process more energetically feasible and cost-effective.

2. Solution Formation: While cryolite's primary purpose is to lower the melting point, it also acts as a solvent, allowing alumina to dissolve within the molten cryolite. This creates a fluid mixture that aids in the efficient conduction of electricity during electrolysis.

3. Electrical Conductivity: Cryolite enhances the electrical conductivity of the solution, facilitating more efficient passage of electric current required for aluminum extraction.

4. Correct Choice: From the given options, the primary reason for adding cryolite is to "Lower the melting point of alumina."

Answer 2

Therefore, the most accurate answer is: Option 1: Lower the melting point of alumina

The electrolysis of alumina (Al₂O₃) to obtain aluminum metal requires a high temperature due to alumina's high melting point. Cryolite (Na₃AlF₆) is added to the process for a specific reason.

Let's analyze each option:

• Option 1: Lower the melting point of alumina: This is correct. Pure alumina has a very high melting point (around 2072°C), which makes its electrolysis economically unfeasible. Cryolite acts as a flux, lowering the melting point of alumina to a more manageable temperature (around 900-950°C).
• Option 2: Dissolve the alumina in molten cryolite: This is also correct. Cryolite forms a molten solution with alumina, allowing the ions to move freely, which is essential for electrolysis.
• Option 3: Remove impurities from alumina: While some purification might occur during the process, this is not the primary function of cryolite.
• Option 4: Increase electrical conductivity: This is also correct. By dissolving alumina in molten cryolite, the ionic conductivity of the solution is increased, facilitating the electrolysis process.

However, the main reason cryolite is added is to lower the melting point of alumina, which then results in the other benefits.

Therefore, while options 2 and 4 are correct effects, option 1 is the main reason why cryolite is used.