The best method to separate a mixture of ortho and para nitrophenol `(1 : 1)` is :

Options

A. Steam distillation
B. Crystallisation
C. Vaporisation
D. Spectroscopy

Explanation

To separate a mixture of ortho and para nitrophenol (1:1), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structures**:
- Identify the structures of ortho-nitrophenol and para-nitrophenol.
- Ortho-nitrophenol has the -OH (hydroxyl) group and the -NO2 (nitro) group adjacent to each other, while para-nitrophenol has these groups opposite each other on the benzene ring.

2. **Analyze Physical Properties**:
- Note that ortho-nitrophenol exhibits intramolecular hydrogen bonding due to the proximity of the -OH and -NO2 groups.
- This intramolecular hydrogen bonding affects its physical properties, particularly its melting point and boiling point.

3. **Compare Boiling Points**:
- The presence of intramolecular hydrogen bonding in ortho-nitrophenol results in a lower boiling point compared to para-nitrophenol.
- This difference in boiling points is crucial for separation.

4. **Choose Separation Method**:
- Since there is a significant difference in boiling points, steam distillation is an effective method for separation.
- In steam distillation, the compound with the lower boiling point (ortho-nitrophenol) will vaporize first, allowing it to be collected separately from para-nitrophenol.

5. **Perform Steam Distillation**:
- Set up the steam distillation apparatus, ensuring that the mixture of ortho and para-nitrophenol is heated.
- As the mixture is heated, the ortho-nitrophenol will vaporize and can be collected in the distillate, while para-nitrophenol remains in the distillation flask.

6. **Collect and Analyze**:
- Collect the distillate containing ortho-nitrophenol and analyze it to confirm the separation.
- The remaining para-nitrophenol can be collected from the distillation flask.

### Final Answer:
The best method to separate a mixture of ortho and para nitrophenol (1:1) is **steam distillation**.