Which of the following compounds does magnesium precipitate when you test for it?

Options

A. `MgCO_(3)*MgO`
B. `MgCO_(3)`
C. `Mg(OH)_(2)`
D. `MgNO_(4)PO_(4)*6H_(2)O`

Answer

To determine which compound magnesium precipitates when tested, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Group of Magnesium**:
- Magnesium is classified as a sixth group basic radical in qualitative inorganic analysis.

2. **Identify the Reagents Used for Testing**:
- The reagents used to test for magnesium in the sixth group are ammonium hydroxide (NH4OH) and sodium hydrogen phosphate (Na2HPO4).

3. **Conduct the Reaction**:
- When magnesium ions (Mg²⁺) react with ammonium hydroxide (NH4OH) and sodium hydrogen phosphate (Na2HPO4), a precipitate is formed.

4. **Identify the Precipitate**:
- The reaction produces magnesium ammonium phosphate, which can be represented as MgNH4PO4·6H2O. This compound is a white precipitate.

5. **Select the Correct Option**:
- From the given options, the compound that magnesium precipitates when tested is MgNH4PO4·6H2O, which corresponds to option 4.

### Final Answer:
- The compound that magnesium precipitates when tested is **MgNH4PO4·6H2O** (option 4).