`H_3 BO_3 overset("Red heat")to X`. 'X' in the reaction is

Options

A. `H_2 B_4 O_7`
B. `HBO_2`
C. `B_2 O_3`
D. B

Answer

To solve the question regarding the thermal decomposition of boric acid (H₃BO₃) when subjected to red heat, we can break down the reaction step by step.

### Step-by-Step Solution:

1. **Identify the Starting Material**:
The starting material in this reaction is boric acid, which has the chemical formula H₃BO₃.

2. **First Heating Stage**:
When H₃BO₃ is heated to about 100°C, it undergoes dehydration to form metaboric acid (HBO₂):
\[ \text{H}_3\text{BO}_3 \xrightarrow{100°C} \text{HBO}_2 + \text{H}_2\text{O} \]

3. **Second Heating Stage**:
If metaboric acid (HBO₂) is further heated to around 160°C, it can dehydrate again to form tetraboric acid (H₂B₄O₇):
\[ \text{HBO}_2 \xrightarrow{160°C} \text{H}_2\text{B}_4\text{O}_7 + \text{H}_2\text{O} \]

4. **Final Heating Stage**:
When tetraboric acid (H₂B₄O₇) is heated to red heat, it decomposes to form boron trioxide (B₂O₃) and water:
\[ \text{H}_2\text{B}_4\text{O}_7 \xrightarrow{\text{Red Heat}} \text{B}_2\text{O}_3 + \text{H}_2\text{O} \]

5. **Conclusion**:
Therefore, when H₃BO₃ is heated to red heat, the final product 'X' formed is boron trioxide (B₂O₃), also known as boric anhydride.

### Final Answer:
The substance 'X' in the reaction is **B₂O₃** (boron trioxide).

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