Ortho phosphoric acid on heating above `300^(@)C` gives :

Options

A. hypophosphorus acid
B. hypophosphoric acid
C. metaphosphoric acid
D. phosphorous acid

Answer

To solve the question regarding the thermal decomposition of orthophosphoric acid (H₃PO₄) when heated above 300°C, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Compound**:
- The compound in question is orthophosphoric acid, which has the chemical formula H₃PO₄.

2. **Understand the Heating Process**:
- When orthophosphoric acid is heated to temperatures above 300°C, it undergoes thermal decomposition.

3. **Write the Decomposition Reaction**:
- The thermal decomposition of H₃PO₄ can be represented by the following reaction:
\[ 4 \text{H}_3\text{PO}_4 \rightarrow \text{P}_4\text{O}_{10} + 6 \text{H}_2\text{O} \]
- This indicates that four moles of orthophosphoric acid decompose to form one mole of tetraphosphorus decoxide (P₄O₁₀) and six moles of water (H₂O).

4. **Formation of Metaphosphoric Acid**:
- The tetraphosphorus decoxide (P₄O₁₀) can further react with water to form metaphosphoric acid (HPO₃):
\[ \text{P}_4\text{O}_{10} + 2 \text{H}_2\text{O} \rightarrow 4 \text{HPO}_3 \]
- This indicates that one mole of P₄O₁₀ reacts with two moles of water to produce four moles of metaphosphoric acid.

5. **Final Products**:
- Therefore, the final products of heating orthophosphoric acid above 300°C are P₄O₁₀ and HPO₃ (metaphosphoric acid).

6. **Conclusion**:
- The correct answer to the question is that orthophosphoric acid on heating above 300°C gives tetraphosphorus decoxide (P₄O₁₀) and metaphosphoric acid (HPO₃).