..........is added to ` Al_2O_3 ` in Hall - Heroult process for extraction of aluminium to ..............the melting point and to ...............the electrical conductivity of the melt.

Answer

To answer the question regarding the Hall-Héroult process for the extraction of aluminum, we can break it down into the following steps:

### Step 1: Identify the compound added to Al₂O₃
In the Hall-Héroult process, **cryolite (Na₃AlF₆)** is added to Al₂O₃.

### Step 2: Explain the purpose of adding cryolite
The addition of cryolite serves two main purposes:
1. **To lower the melting point**: The melting point of pure Al₂O₃ is quite high (around 2050°C). By adding cryolite, the melting point of the mixture is significantly reduced, making the process more energy-efficient.
2. **To enhance electrical conductivity**: Cryolite increases the electrical conductivity of the molten mixture, which is essential for the electrolysis process to occur efficiently.

### Final Answer
Cryolite (Na₃AlF₆) is added to Al₂O₃ in the Hall-Héroult process for the extraction of aluminum to **lower the melting point** and to **enhance the electrical conductivity** of the melt.

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