Assertion (A) : Silica is soluble in ` HF`.
Reason (R) : `SiO_2 + 4HF rarr SiF_4 +2H_2 O`
` SiF_4 + 2HF rarr H_2 SiF_6` .

Options

A. If both (A) and (R) are correct and (R) is correct explanation of (A)
B. If both (A) and (R) are correct and (R) is not correct explanation of (A)
C. If (A) is correct , but (R) is incorrect
D. If (A) is incorrect but (R) is correct .

Answer

To solve the assertion and reason question regarding the solubility of silica in hydrofluoric acid (HF), we can break it down into clear steps:

### Step 1: Understand the Assertion
The assertion states that "Silica is soluble in HF." Silica, chemically known as silicon dioxide (SiO2), is indeed known to react with hydrofluoric acid.

**Hint:** Recall the properties of silica and its interactions with acids.

### Step 2: Analyze the Reason
The reason provided is the chemical reaction:
\[ \text{SiO}_2 + 4\text{HF} \rightarrow \text{SiF}_4 + 2\text{H}_2\text{O} \]
This reaction indicates that silica reacts with hydrofluoric acid to produce silicon tetrafluoride (SiF4) and water (H2O).

**Hint:** Consider what products are formed when silica reacts with HF.

### Step 3: Further Reaction of SiF4
The second part of the reason states:
\[ \text{SiF}_4 + 2\text{HF} \rightarrow \text{H}_2\text{SiF}_6 \]
This shows that silicon tetrafluoride can further react with hydrofluoric acid to form hexafluorosilicic acid (H2SiF6).

**Hint:** Think about the significance of the products formed in the context of solubility.

### Step 4: Conclusion
Both the assertion and the reason are correct. The reaction of silica with HF leads to the formation of soluble compounds (SiF4 and H2SiF6), confirming that silica is indeed soluble in hydrofluoric acid.

**Hint:** Reflect on how the formation of soluble products supports the assertion.

### Final Answer
Both the assertion and reason are correct, and the reason provides a valid explanation for the assertion.