Assertion (A) : When `CO_2` is passed through lime water the solution turns milky but with an excess of `CO_2` the solution becomes clear again .
Reason (R) : Excess `CO_2` changes the suspension to a colloidal solution .

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 .

Explanation

To solve the assertion and reason question, we will analyze both the assertion (A) and the reason (R) step by step.

### Step 1: Understand the Assertion
The assertion states that when carbon dioxide (CO₂) is passed through lime water (which is a solution of calcium hydroxide, Ca(OH)₂), the solution turns milky. This happens due to the formation of calcium carbonate (CaCO₃), which is a white precipitate.

**Hint:** Recall that lime water is a saturated solution of calcium hydroxide.

### Step 2: Reaction of CO₂ with Lime Water
When CO₂ is bubbled through lime water, it reacts with calcium hydroxide to form calcium carbonate:
\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O} \]
The formation of CaCO₃ causes the solution to appear milky.

**Hint:** Identify the products formed during the reaction and their physical states.

### Step 3: Understand the Effect of Excess CO₂
The assertion also mentions that with an excess of CO₂, the solution becomes clear again. This occurs because the calcium carbonate formed can react further with CO₂ and water to form calcium bicarbonate (Ca(HCO₃)₂), which is soluble in water:
\[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 \]
The formation of this soluble compound causes the milky solution to become clear.

**Hint:** Consider how the solubility of the products affects the appearance of the solution.

### Step 4: Analyze the Reason
The reason states that excess CO₂ changes the suspension to a colloidal solution. However, this statement is incorrect. The clarity of the solution is due to the formation of soluble calcium bicarbonate, not because it becomes a colloidal solution.

**Hint:** Distinguish between a colloidal solution and a true solution based on particle size and solubility.

### Step 5: Conclusion
Based on the analysis:
- The assertion (A) is correct because CO₂ does cause lime water to turn milky and then clear with excess CO₂.
- The reason (R) is incorrect because the clarity is due to the formation of a soluble compound, not a colloidal solution.

Thus, the correct answer is that the assertion is correct, but the reason is incorrect.

**Final Answer:** Assertion is correct, Reason is incorrect (Option C).