Select the correct statements for Ellingham diagram.

Options

A. The slope of the curves of the formation of metal oxide is `+ve` because `Delta_(f) G^(Ө)` becomes less negative or increases with the rise in temperature.
B. Each curve is a straight line except when some change takes place in phase `(s rarr 1 or 1 rarr g)`
C. Each curve is not a straight line except when some changes takes place in phase `(s rarr 1 or 1 rarr g)`.
D. The slope of the curves of the formation of metal oxide is `-ve` becomes less negative or increases with the rise in temperature.

Answer

To solve the question regarding the correct statements for the Ellingham diagram, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Understanding the Ellingham Diagram**:
The Ellingham diagram is a graphical representation that shows the temperature dependence of the stability of compounds, particularly metal oxides. It plots the Gibbs free energy change (ΔG) for the formation of these oxides against temperature.

2. **Analyzing Statement A**:
- **Statement**: "The slope of the curves for the formation of metal oxides is positive because ΔG° becomes less negative."
- **Analysis**: The slope of the curves in the Ellingham diagram is actually negative. As temperature increases, ΔG becomes less negative (or more positive), but the slope itself is negative because the y-axis (ΔG) is negative.
- **Conclusion**: This statement is **incorrect**.

3. **Analyzing Statement B**:
- **Statement**: "Each curve is a straight line except when some changes take place in the phase."
- **Analysis**: This statement is true. Most curves in the Ellingham diagram are straight lines, but they can curve at points where phase changes occur (e.g., solid to liquid).
- **Conclusion**: This statement is **correct**.

4. **Analyzing Statement C**:
- **Statement**: "Each curve is not a straight line except when some changes take place."
- **Analysis**: This statement is incorrect. Most curves are straight lines, and they only deviate from being straight during phase changes.
- **Conclusion**: This statement is **incorrect**.

5. **Analyzing Statement D**:
- **Statement**: "The slopes of the curves for the formation of metal oxides are negative."
- **Analysis**: This statement is correct. The slopes are indeed negative, reflecting that as temperature increases, the Gibbs free energy becomes less negative.
- **Conclusion**: This statement is **correct**.

### Final Answer:
The correct statements regarding the Ellingham diagram are **B and D**.