An alkaline earth metal, sulphate is soluble in water its hydroxide is not soluble in water and its oxide does not form rock salt structure, then metal is:

Options

A. Be
B. Mg
C. CA
D. Sr

Answer

To solve the problem, we need to analyze the properties of the alkaline earth metals and their compounds based on the information given in the question.

### Step-by-Step Solution:

1. **Identify the Properties of Alkaline Earth Metals**:
- The alkaline earth metals include Beryllium (Be), Magnesium (Mg), Calcium (Ca), and Strontium (Sr).
- Their sulfates, hydroxides, and oxides exhibit different solubility and structural properties.

2. **Analyze the Sulfate Solubility**:
- The question states that the sulfate of the metal is soluble in water.
- Generally, the solubility of alkaline earth metal sulfates decreases as we move down the group from Beryllium to Strontium.
- Beryllium sulfate (BeSO₄) is known to be soluble in water, while Magnesium sulfate (MgSO₄) is also soluble, but Calcium sulfate (CaSO₄) and Strontium sulfate (SrSO₄) are less soluble.

3. **Analyze the Hydroxide Solubility**:
- The question states that the hydroxide of the metal is not soluble in water.
- Hydroxides of alkaline earth metals become more soluble as we move down the group.
- Beryllium hydroxide (Be(OH)₂) is amphoteric and is not soluble in water, while Magnesium hydroxide (Mg(OH)₂) is slightly soluble, and Calcium hydroxide (Ca(OH)₂) is more soluble.

4. **Analyze the Oxide Structure**:
- The question mentions that the oxide does not form a rock salt structure.
- The oxides of alkaline earth metals typically have different structures. Beryllium oxide (BeO) does not adopt the rock salt structure, while Magnesium oxide (MgO), Calcium oxide (CaO), and Strontium oxide (SrO) do.

5. **Conclusion**:
- Based on the analysis:
- Beryllium sulfate is soluble in water.
- Beryllium hydroxide is not soluble in water.
- Beryllium oxide does not form a rock salt structure.
- Therefore, the metal in question is **Beryllium (Be)**.

### Final Answer:
The metal is **Beryllium (Be)**.