Incorrect statement in electrolysis of `Al_(2)O_(3)` by Hall-Heroult process is :

Options

A. Cryolite `Na_(3)[AlF_(6)]` lowers the m.pt. of `Al_(2)O_(3)` and increases its electrical conductivity
B. Al is obtained at cathode and `CO_(2)` at anode
C. `Li_(2)CO_(3)` can be used in place of cryolite `(Na_(3)AlF_(6))`
D. `MgF_(2)` can be used in place of flourspare `(CaF_(2))`

Explanation

To solve the question regarding the incorrect statement in the electrolysis of Al₂O₃ by the Hall-Heroult process, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understanding the Hall-Heroult Process:**
- The Hall-Heroult process is used for the extraction of aluminum from its oxide, Al₂O₃.
- This process involves the electrolysis of molten Al₂O₃, typically mixed with cryolite (Na₃AlF₆) and sometimes CaF₂ to lower the melting point and increase conductivity.

2. **Analyzing the Statements:**
- **Statement 1:** "Cryolite that is Na₃AlF₆ lowers the melting point of Al₂O₃ and increases its electrical conductivity."
- **Analysis:** This statement is true. Cryolite is indeed used to lower the melting point of Al₂O₃ and enhance its conductivity.

- **Statement 2:** "Al is obtained at cathode and CO₂ at anode."
- **Analysis:** This statement is also true. During electrolysis, aluminum is deposited at the cathode, and carbon dioxide is produced at the anode from the oxidation of oxide ions.

- **Statement 3:** "Li₂CO₃ can be used in place of Cryolite."
- **Analysis:** This statement is true as well. Lithium carbonate can serve a similar function as cryolite in the process.

- **Statement 4:** "MgF₂ can be used in place of CaF₂."
- **Analysis:** This statement is incorrect. If MgF₂ is used instead of CaF₂, magnesium ions (Mg²⁺) will preferentially discharge at the cathode due to their higher reduction potential compared to aluminum ions (Al³⁺). This would result in the production of magnesium instead of aluminum, which is not the desired outcome.

3. **Conclusion:**
- The incorrect statement regarding the electrolysis of Al₂O₃ by the Hall-Heroult process is **Statement 4**: "MgF₂ can be used in place of CaF₂."

### Final Answer:
The incorrect statement in the electrolysis of Al₂O₃ by the Hall-Heroult process is: **"MgF₂ can be used in place of CaF₂."**

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