A : `Na_(2) S_(2) O_3` on reaction with `l_(2)` gives `Na_(2) S_(4) O_(6)` .
R : This reaction involves colour and electronic change both .

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion , then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion , then mark (2)
C. If Assertion is true statement but Reason is false , then mark (3)
D. If both Assertion and Reason are false statements , then mark (4)

Answer

To solve the assertion-reason question regarding the reaction of sodium thiosulfate (`Na2S2O3`) with iodine (`I2`), we will break down the problem step by step.

### Step 1: Write the Reaction
The reaction between sodium thiosulfate and iodine can be represented as:
\[ \text{Na}_2\text{S}_2\text{O}_3 + \text{I}_2 \rightarrow \text{Na}_2\text{S}_4\text{O}_6 + 2\text{I}^- \]

### Step 2: Determine Oxidation States
Next, we need to determine the oxidation states of sulfur in both reactants and products to understand the redox nature of the reaction.

1. **For `Na2S2O3`:**
- Let the oxidation state of sulfur be \( x \).
- The equation for the oxidation state is:
\[ 2x + 2(+1) + 3(-2) = 0 \]
Simplifying this gives:
\[ 2x + 2 - 6 = 0 \implies 2x - 4 = 0 \implies x = +2 \]
Thus, the oxidation state of sulfur in `Na2S2O3` is \( +2 \).

2. **For `Na2S4O6`:**
- Let the oxidation state of sulfur in `Na2S4O6` be \( y \).
- The equation for the oxidation state is:
\[ 4y + 2(+1) + 6(-2) = 0 \]
Simplifying this gives:
\[ 4y + 2 - 12 = 0 \implies 4y - 10 = 0 \implies y = +2.5 \]
Thus, the oxidation state of sulfur in `Na2S4O6` is \( +2.5 \).

### Step 3: Identify the Redox Process
In this reaction:
- Sulfur is oxidized from \( +2 \) in `Na2S2O3` to \( +2.5 \) in `Na2S4O6`.
- Iodine is reduced from \( 0 \) in `I2` to \( -1 \) in `I^-`.

This confirms that the reaction is indeed a redox reaction.

### Step 4: Analyze Color Change
- `Na2S2O3` is typically colorless or white.
- The product `Na2S4O6` and the iodide ion (`I^-`) can impart a brown color to the solution.

### Conclusion
- **Assertion (A)**: The statement that `Na2S2O3` reacts with `I2` to form `Na2S4O6` is correct.
- **Reason (R)**: The reaction involves both color and electronic changes, which is also correct.

However, while both statements are correct, the reason provided does not adequately explain the assertion. Therefore, the assertion is true, the reason is true, but the reason is not the correct explanation for the assertion.

### Final Answer
- Assertion (A) is true.
- Reason (R) is true, but it is not the correct explanation for the assertion.