Each pair forms ideal solution except

Options

A. `C_(2)H_(5)Br` and `C_(2)H_(5)I`
B. `C_(2)H_(5)Cl` and `C_(2)H_(5)Br`
C. `C_(6)H_(6)` and `C_(6)H_(5)CH_(3)`
D. `C_(2)H_(6)I` and `C_(2)H_(5)OH`

Explanation

To solve the question "Each pair forms ideal solution except," we need to analyze the given pairs of compounds and determine whether they form ideal solutions or not. An ideal solution is one where the enthalpy of mixing (ΔH) is zero and the volume change on mixing (ΔV) is also zero.

### Step-by-Step Solution:

1. **Identify the Pairs**: The question provides four pairs of compounds. We will analyze each pair to see if they form an ideal solution.

2. **Pair 1: Ethyl Bromide (C2H5Br) and Ethyl Iodide (C2H5I)**:
- Both compounds share the common ethyl group (C2H5).
- Since they have a common component, the interactions between the molecules will be similar.
- Therefore, ΔH (enthalpy change) and ΔV (volume change) will be close to zero.
- **Conclusion**: This pair forms an ideal solution.

3. **Pair 2: Chloroethane (C2H5Cl) and Bromoethane (C2H5Br)**:
- Again, both compounds share the common ethyl group (C2H5).
- The difference in halogen atoms (Cl and Br) does not significantly affect the interactions.
- Therefore, ΔH and ΔV will also be near zero.
- **Conclusion**: This pair forms an ideal solution.

4. **Pair 3: Benzene (C6H6) and Toluene (C6H5CH3)**:
- Both compounds have a common benzene ring.
- The presence of a methyl group in toluene does not significantly alter the interactions.
- Thus, ΔH and ΔV will be close to zero.
- **Conclusion**: This pair forms an ideal solution.

5. **Pair 4: Iodoethane (C2H5I) and Ethanol (C2H5OH)**:
- Ethanol has an -OH group which can form hydrogen bonds.
- The presence of hydrogen bonding between ethanol and iodoethane leads to significant interactions that deviate from ideal behavior.
- Therefore, ΔH and ΔV will not be zero due to these strong intermolecular forces.
- **Conclusion**: This pair does not form an ideal solution.

### Final Answer:
The pair that does not form an ideal solution is **Iodoethane and Ethanol**.