Why `Al` can reduce oxide of `Cr` but not reduce oxide of `Mg` and `Ca` at `800^(@)C`?

Explanation

To understand why aluminum (Al) can reduce the oxide of chromium (Cr) but not the oxides of magnesium (Mg) and calcium (Ca) at 800°C, we can analyze the situation using the Ellingham diagram, which provides insight into the thermodynamic stability of metal oxides.

### Step-by-Step Solution:

1. **Understanding the Ellingham Diagram**:
The Ellingham diagram plots the Gibbs free energy change (ΔG) for the formation of oxides of various metals against temperature. The lower the curve on the diagram, the more stable the oxide is at that temperature.

2. **Identifying the Position of Metals**:
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