Which of the following reaction(s) relevat t the microcosmic salt bead test?

Options

A. `Cr_(2)O_(3)+3B_(2)O_(3) to 2Cr(BO_(2))_(3)`
B. `Co O+ZnO to CoZnO_(2)`
C. `CoO+NaPO_(4)toNaCoPO_(4)`
D. `Al_(2)(SO_(4))_(3)+3Na_(2)CO_(3) to Al_(2)O_(3)+3Na_(2)SO_(4)+3CO_(2)`

Answer

To determine which of the given reactions are relevant to the microcosmic salt bead test, we first need to understand the test itself and the reactions involved.

### Step-by-Step Solution:

1. **Understand the Microcosmic Salt Bead Test**:
- The microcosmic salt bead test is used for the identification of colored cations. The microcosmic salt used in this test is sodium ammonium phosphate (NaNH4PO4·4H2O).
- When heated, this salt decomposes to form sodium metaphosphate (NaPO3), water vapor, and ammonia gas.

2. **Identify the Reactions Involved**:
- The primary reaction of interest is the interaction of sodium metaphosphate (NaPO3) with various oxides to form different products.

3. **Evaluate Each Given Reaction**:
- **Reaction 1**: Cr2O3 + B2O3 → CrBO2 (twice)
- This reaction does not involve sodium metaphosphate and is not relevant to the microcosmic salt bead test.
- **Reaction 2**: Cobalt oxide + Zinc oxide → CoZnO2
- This reaction does not involve sodium metaphosphate and is not relevant to the microcosmic salt bead test.
- **Reaction 3**: Cobalt oxide + NaPO4 → NaCoPO4
- This reaction involves sodium metaphosphate (NaPO3) and cobalt oxide, which is relevant to the microcosmic salt bead test.
- **Reaction 4**: Aluminum sulfate + Sodium carbonate → Aluminum oxide + Sodium sulfate + Carbon dioxide
- This reaction does not involve sodium metaphosphate and is not relevant to the microcosmic salt bead test.

4. **Conclusion**:
- Among the given reactions, only the third reaction (Cobalt oxide + NaPO4 → NaCoPO4) is relevant to the microcosmic salt bead test.

### Final Answer:
The reaction relevant to the microcosmic salt bead test is:
- Cobalt oxide + NaPO4 → NaCoPO4 (Option 3).