When orthoboric acid `(H_(3)BO_(3))` is heated, the residue is

Options

A. boron
B. metaboric acid and hydrogen
C. boric anhydride
D. borax

Explanation

To determine the residue when orthoboric acid (H₃BO₃) is heated, we can follow these steps:

### Step 1: Heating Orthoboric Acid
When orthoboric acid (H₃BO₃) is heated, it undergoes a dehydration reaction.

### Step 2: Formation of Metaboric Acid
At around 170 °C (or 443 K), orthoboric acid loses a molecule of water (H₂O) and converts to metaboric acid (HBO₂). The reaction can be represented as:
\[ H_3BO_3 \rightarrow HBO_2 + H_2O \]

### Step 3: Further Heating of Metaboric Acid
If metaboric acid (HBO₂) is heated further, it can dehydrate again, leading to the formation of tetraboric acid (H₄B₂O₇) along with the release of more water:
\[ 2 HBO_2 \rightarrow H_4B_2O_7 + H_2O \]

### Step 4: Final Heating to Boron Trioxide
When tetraboric acid (H₄B₂O₇) is heated to higher temperatures (around 370 °C or 643 K), it decomposes to form boron trioxide (B₂O₃), also known as boric anhydride:
\[ H_4B_2O_7 \rightarrow B_2O_3 + H_2O \]

### Conclusion
The final residue after heating orthoboric acid is boron trioxide (B₂O₃).

### Answer
The correct answer is boron trioxide (B₂O₃).

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