Choose the incorrect statement out of the following .

Options

A. LiCl and `MgCl_2` are soluble in ethanol
B. LiCl and `MgCl_2` are deliquescent and crystallised from aqueous solution as hydrates
C. `Li_2O` and MgO combines with excess oxygen to produce superoxide
D. carbonates of Li and Mg decomposes easily on heating to from oxides and `CO_2`

Answer

To solve the question of identifying the incorrect statement regarding Lithium (Li) and Magnesium (Mg), we will analyze each statement provided in the options.

### Step-by-Step Solution:

1. **Understand the Diagonal Relationship**:
- Lithium and Magnesium exhibit a diagonal relationship in the periodic table, meaning they share some similar properties due to their similar size and charge density.

2. **Evaluate Each Statement**:
- **Option 1**: "LiCl and MgCl2 are soluble in ethanol."
- **Analysis**: Both LiCl and MgCl2 are indeed soluble in ethanol. This statement is **correct**.

- **Option 2**: "LiCl and MgCl2 are deliquescent and crystallize in the form of hydrates."
- **Analysis**: LiCl is known to be deliquescent and can crystallize as LiCl·2H2O, while MgCl2 can crystallize as MgCl2·8H2O. This statement is **correct**.

- **Option 3**: "Li2O and MgO combine with excess oxygen to produce superoxide."
- **Analysis**: Superoxides are stable with larger cations. Lithium oxide (Li2O) and Magnesium oxide (MgO) do not form superoxides when combined with excess oxygen. This statement is **incorrect**.

- **Option 4**: "Carbonates of Lithium and Magnesium decompose easily on heating to form oxides and CO2."
- **Analysis**: Lithium carbonate (Li2CO3) and Magnesium carbonate (MgCO3) decompose upon heating to produce their respective oxides and carbon dioxide. This statement is **correct**.

3. **Conclusion**:
- After evaluating all the statements, the incorrect statement is **Option 3**: "Li2O and MgO combine with excess oxygen to produce superoxide."

### Final Answer:
The incorrect statement is **Option 3**.