When `H_2S` has is passed through an orange red solution (X), the solution turns milky. When an alkali is added to this orange red solution it turns yellow and on acidifying this yellow solution again turns orange red. X is

Options

A. `K_2CrO_4`
B. `KMnO_4`
C. `Na_2Cr_2O_7`
D. `K_2Cr_2O_7`

Explanation

To solve the problem step by step, we will analyze the reactions and color changes described in the question.

### Step 1: Identify the initial orange-red solution (X)
The orange-red solution mentioned in the question is likely potassium dichromate (K₂Cr₂O₇). This compound is known for its distinct orange-red color in solution.

**Hint:** Look for compounds that are known to have a characteristic orange-red color in solution.

### Step 2: Reaction with H₂S
When hydrogen sulfide (H₂S) is passed through the orange-red solution of potassium dichromate, a reaction occurs. H₂S is a reducing agent and can reduce the dichromate ions (Cr₂O₇²⁻) to chromium(III) ions (Cr³⁺), which are typically colorless or light green. The reduction process also leads to the formation of sulfur (S) as a precipitate, which makes the solution turn milky.

**Hint:** Consider the reducing properties of H₂S and what it can do to dichromate ions.

### Step 3: Addition of alkali
When an alkali (like KOH) is added to the resulting solution, the solution turns yellow. This is because the chromium(III) ions can react with hydroxide ions to form chromium(III) hydroxide, which can further react to form potassium chromate (K₂CrO₄), which is yellow in color.

**Hint:** Think about how the addition of hydroxide ions can affect the color of chromium solutions.

### Step 4: Acidification of the yellow solution
When the yellow solution (potassium chromate) is acidified (for example, with hydrochloric acid, HCl), it converts back to potassium dichromate. The reaction is as follows:
\[ \text{2 K}_2\text{CrO}_4 + 2 \text{HCl} \rightarrow \text{K}_2\text{Cr}_2\text{O}_7 + 2 \text{KCl} + \text{H}_2\text{O} \]
This reaction restores the orange-red color of the solution.

**Hint:** Consider how acid-base reactions can reverse the changes caused by previous steps.

### Conclusion
The compound X that fits all these observations is potassium dichromate (K₂Cr₂O₇).

**Final Answer:** The compound X is potassium dichromate (K₂Cr₂O₇).