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 alkaline earth metals and their compounds based on the given conditions:

1. **Identify the alkaline earth metals**: The alkaline earth metals include Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), and Barium (Ba).

2. **Analyze the solubility of sulfates**:
- The solubility of sulfates of alkaline earth metals generally decreases as we move down the group.
- Beryllium sulfate (BeSO₄) is soluble in water, while the solubility of magnesium sulfate (MgSO₄) is also good, but it is less soluble than beryllium sulfate. Calcium sulfate (CaSO₄) is moderately soluble, while strontium sulfate (SrSO₄) and barium sulfate (BaSO₄) are largely insoluble.

3. **Analyze the solubility of hydroxides**:
- The hydroxides of alkaline earth metals become more soluble as we move down the group.
- Beryllium hydroxide (Be(OH)₂) is amphoteric and has low solubility in water. Magnesium hydroxide (Mg(OH)₂) is also poorly soluble. Calcium hydroxide (Ca(OH)₂) is more soluble than both beryllium and magnesium hydroxides, and strontium hydroxide (Sr(OH)₂) and barium hydroxide (Ba(OH)₂) are quite soluble.

4. **Analyze the structure of oxides**:
- The oxides of alkaline earth metals generally form a rock salt structure, which is characteristic of ionic compounds. However, beryllium oxide (BeO) is known to have a covalent character and does not form a rock salt structure.

5. **Summarize the findings**:
- Beryllium sulfate (BeSO₄) is soluble in water.
- Beryllium hydroxide (Be(OH)₂) is not soluble in water.
- Beryllium oxide (BeO) does not form a rock salt structure.

Based on the analysis, the metal that fits all the criteria given in the question is **Beryllium (Be)**.

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