Gas that turns lime water milky and aciddied `K_(2)Cr_(2)O_(7)` green is (a) ____.

Answer

To solve the question, we need to identify the gas that can turn lime water milky and also change the color of acidified potassium dichromate (K2Cr2O7) from orange to green.

### Step-by-Step Solution:

1. **Understanding Lime Water Reaction**:
- Lime water is a dilute solution of calcium hydroxide (Ca(OH)2).
- When certain gases react with lime water, they can form insoluble compounds that cause the solution to turn milky.

2. **Identifying the Gas for Lime Water**:
- The gas that is commonly known to turn lime water milky is sulfur dioxide (SO2).
- The reaction is as follows:
\[ \text{Ca(OH)}_2 + \text{SO}_2 \rightarrow \text{CaSO}_3 (s) + \text{H}_2O \]
- Calcium sulfite (CaSO3) is formed, which is insoluble in water and gives the milky appearance.

3. **Understanding the Reaction with Acidified K2Cr2O7**:
- Potassium dichromate (K2Cr2O7) is orange in color.
- When sulfur dioxide is bubbled through an acidified solution of potassium dichromate, it reduces the dichromate ions (Cr2O7^2-) to chromium(III) ions (Cr^3+), which are green in color.
- The reaction can be represented as:
\[ \text{Cr}_2\text{O}_7^{2-} + \text{SO}_2 + \text{H}^+ \rightarrow \text{Cr}^{3+} + \text{K}_2\text{SO}_4 + \text{H}_2\text{O} \]

4. **Conclusion**:
- The gas that turns lime water milky and acidified K2Cr2O7 green is sulfur dioxide (SO2).

### Final Answer:
The gas that turns lime water milky and acidified K2Cr2O7 green is **(a) Sulfur Dioxide (SO2)**.

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