When a fluoride is heated with concentration `H_(2)SO_(4)` in a glass tube and if a drop of water is held at the mouth of the glass tube, a white deposit formed is of

Options

A. `H_(2)SiF_(6)`
B. `SiO_(6)`
C. `H_(2)SiO_(3)`
D. `SiF_(4)+H_(2)F_(2)`

Answer

To solve the question, we will go through the reactions step by step to determine the white deposit formed when a fluoride is heated with concentrated sulfuric acid (H₂SO₄) and a drop of water is held at the mouth of the glass tube.

### Step-by-Step Solution:

1. **Identify the Fluoride**:
Let's consider calcium fluoride (CaF₂) as the fluoride that is heated with concentrated H₂SO₄.

2. **Reaction with Concentrated H₂SO₄**:
When calcium fluoride is heated with concentrated sulfuric acid, the reaction can be written as:
\[ \text{CaF}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + 2\text{HF} \]
Here, calcium sulfate (CaSO₄) is formed as a solid, and hydrogen fluoride (HF) is released as a gas.

3. **Interaction with Glass**:
The glass tube is made of silicon dioxide (SiO₂). The hydrogen fluoride gas produced reacts with the glass:
\[ 2\text{HF} + \text{SiO}_2 \rightarrow \text{SiF}_4 + \text{H}_2\text{O} \]
In this reaction, silicon tetrafluoride (SiF₄) gas and water (H₂O) are produced.

4. **Reaction with Water**:
The silicon tetrafluoride gas that escapes reacts with the drop of water at the mouth of the glass tube:
\[ \text{SiF}_4 + 2\text{H}_2\text{O} \rightarrow \text{H}_2\text{SiO}_3 + 4\text{HF} \]
This reaction produces metasilicic acid (H₂SiO₃), which is a solid, and hydrogen fluoride gas.

5. **Formation of White Deposit**:
The metasilicic acid (H₂SiO₃) formed is a solid and will precipitate as a white deposit in the glass tube.

### Conclusion:
The white deposit formed at the mouth of the glass tube is **metasilicic acid (H₂SiO₃)**.