Potassium dichromate is a good oxidizing agent, in the acidic medium the oxidation state of chromium changes by: A. 2B. 3C. 4D. 5

To determine how the oxidation state of chromium changes when potassium dichromate (K\(_2\)Cr\(_2\)O\(_7\)) acts as an oxidizing agent in an acidic medium, follow these steps:

### Step 1: Identify the oxidation states

1. **Oxidation State of Chromium in Potassium Dichromate:**
- Potassium dichromate is K\(_2\)Cr\(_2\)O\(_7\).
- In this compound, potassium (K) has an oxidation state of \(+1\) and oxygen (O) has an oxidation state of \(-2\).
- Let \( x \) be the oxidation state of chromium (Cr). The compound is neutral overall, so the sum of the oxidation states must be zero.

The equation is:
\[
2 \cdot (+1) + 2 \cdot x + 7 \cdot (-2) = 0
\]
Simplify:
\[
2 + 2x - 14 = 0
\]
\[
2x - 12 = 0
\]
\[
x = +6
\]

So, the oxidation state of chromium in K\(_2\)Cr\(_2\)O\(_7\) is \(+6\).

2. **Reduction of Chromium in Acidic Medium:**
- In an acidic medium, potassium dichromate is reduced to chromium(III) ions, Cr\(^{3+}\).
- The oxidation state of chromium in Cr\(^{3+}\) is \(+3\).

### Step 2: Calculate the change in oxidation state

- **Initial Oxidation State:** \(+6\) (in K\(_2\)Cr\(_2\)O\(_7\))
- **Final Oxidation State:** \(+3\) (in Cr\(^{3+}\))

**Change in Oxidation State:**
\[
6 - 3 = 3
\]

### Answer

The oxidation state of chromium changes by \(3\) units in the acidic medium.

**Correct Answer: B. 3**