Colour of `CrO_(4)^(2-)`(aq.) is not changed by

Options

A. dil. HCl
B. `NH_(3)` solution
C. `CH_(3)COOH`
D. `NO_(2)` gas

Explanation

To determine which substance does not change the color of the chromate ion \( \text{CrO}_4^{2-} \), we need to analyze the behavior of chromate ions in different pH environments. Here’s a step-by-step solution:

### Step 1: Understand the Chromate Ion
The chromate ion \( \text{CrO}_4^{2-} \) is typically yellow in color. It can convert to dichromate ion \( \text{Cr}_2\text{O}_7^{2-} \), which is orange, under acidic conditions (pH < 7).

**Hint:** Remember the colors associated with chromate and dichromate ions.

### Step 2: Analyze the Options
We have four options to consider:
1. Hydrochloric acid (HCl)
2. Ammonia solution (NH₃)
3. Acetic acid
4. Nitrogen dioxide (NO₂)

**Hint:** Identify whether each option is acidic or basic.

### Step 3: Evaluate Each Option
- **Option 1: Hydrochloric Acid (HCl)**
- HCl is a strong acid. When added to a solution containing \( \text{CrO}_4^{2-} \), it will lower the pH and convert chromate to dichromate, changing the color from yellow to orange.

**Hint:** Strong acids will typically change the color of chromate ions.

- **Option 2: Ammonia Solution (NH₃)**
- Ammonia is a weak base. In a basic medium (pH > 7), the chromate ion remains yellow and does not convert to dichromate. Therefore, the color does not change.

**Hint:** Basic conditions will keep chromate ions stable.

- **Option 3: Acetic Acid**
- Acetic acid is a weak acid. Similar to HCl, it will lower the pH and convert chromate to dichromate, changing the color from yellow to orange.

**Hint:** Weak acids can also change the color of chromate ions.

- **Option 4: Nitrogen Dioxide (NO₂)**
- NO₂ is a non-metal oxide that can form nitric acid in solution. Therefore, it will also create an acidic environment, changing the color from yellow to orange.

**Hint:** Non-metal oxides generally lead to acidic conditions.

### Step 4: Conclusion
The only option that does not change the color of \( \text{CrO}_4^{2-} \) is **Ammonia Solution (NH₃)**, as it maintains a basic environment where the chromate ion remains yellow.

### Final Answer
The color of \( \text{CrO}_4^{2-} \) is not changed by **Ammonia Solution (NH₃)**.