A colourless gas `X` forms a brown coloured gas when mixed with air. The gas `X` is

Options

A. `N_2 O`
B. `NO`
C. `NH_3`
D. `NO_2`

Explanation

To solve the question, we need to identify the colorless gas `X` that forms a brown-colored gas when mixed with air. Here’s the step-by-step solution:

### Step 1: Identify the characteristics of gas `X`
- The problem states that gas `X` is colorless and forms a brown-colored gas when mixed with air (which contains oxygen).

### Step 2: Consider possible candidates for gas `X`
- We know that several gases can be colorless, but we need to focus on those that can react with oxygen to produce a brown gas.
- Common candidates include nitrogen oxides, particularly nitric oxide (NO) and ammonia (NH3).

### Step 3: Analyze the reaction of nitric oxide (NO)
- Nitric oxide (NO) is a colorless gas.
- When nitric oxide reacts with oxygen (from the air), it forms nitrogen dioxide (NO2), which is a brown gas.

### Step 4: Analyze the reaction of ammonia (NH3)
- Ammonia (NH3) is also a colorless gas, but it does not form a brown gas when mixed with air. Instead, it can react with acids or other substances but does not produce a brown gas.

### Step 5: Conclusion
- Since nitric oxide (NO) is the only gas that fits the description of being colorless and forming a brown gas (nitrogen dioxide, NO2) when mixed with air, we conclude that gas `X` is nitric oxide (NO).

### Final Answer
- The colorless gas `X` is **nitric oxide (NO)**.

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