All questions of Purification and Characterisation of Organic Compounds for NEET Exam

Understanding the Separation Methods
When separating naphthalene and benzoic acid from their mixture, it's essential to consider the physical properties of both compounds. Naphthalene is a non-polar aromatic hydrocarbon, while benzoic acid is a polar compound with acidic properties.
Why Sublimation is the Best Method
Sublimation is a process where a solid turns directly into vapor without passing through the liquid phase. This property is particularly useful for naphthalene, which sublimes at room temperature:
- Naphthalene: Sublimes easily at around 80°C, allowing it to transition to gas without heating to its melting point.
- Benzoic Acid: Has a higher melting point (about 122°C) and does not sublimate under normal conditions.
Separation Process
1. Heating the Mixture: When the mixture is heated, naphthalene will sublime first.
2. Collecting Naphthalene Vapor: The vapor can be collected and cooled, allowing it to solidify separately from benzoic acid.
3. Remaining Benzoic Acid: After naphthalene is removed, benzoic acid can be purified further through recrystallization if necessary.
Advantages of Sublimation
- Efficiency: This method provides a quick and effective way to separate the compounds, as it capitalizes on their differing physical properties.
- Minimal Reagents: Sublimation requires no additional solvents or reagents, making it a cleaner method.
- High Purity: The sublimation process yields high-purity naphthalene, avoiding contamination from solvents.
In conclusion, sublimation is the most suitable method for separating naphthalene and benzoic acid due to the significant differences in their sublimation properties, ensuring efficient and effective separation.

Separation of Phenols and Carboxylic Acids
Separating a mixture of a lighter phenol (A) and an aromatic carboxylic acid (B) can effectively be achieved using a sodium bicarbonate solution. Here's a detailed explanation of why this method works.
Properties of the Compounds
- Phenol (A):
- A weak acid but does not react significantly with sodium bicarbonate.
- Remains soluble in organic solvents and retains its original form in aqueous solutions.
- Aromatic Carboxylic Acid (B):
- A stronger acid compared to phenol.
- Reacts with sodium bicarbonate to form a water-soluble sodium salt and carbon dioxide gas.
Separation Process
1. Dissolution in Sodium Bicarbonate:
- When the mixture is treated with sodium bicarbonate solution, the aromatic carboxylic acid (B) reacts to form a sodium salt of the acid.
- This reaction generates carbon dioxide, which can be observed as effervescence.
2. Formation of Layers:
- The sodium salt of the carboxylic acid is soluble in water, while the phenol remains in the organic phase.
- This allows for the separation of the two compounds into different layers.
3. Extraction:
- The aqueous layer containing the sodium salt can be separated from the organic layer containing phenol.
- The salt can then be converted back to the carboxylic acid by acidifying the aqueous layer.
Conclusion
Using sodium bicarbonate is a selective method for separating lighter phenols from aromatic carboxylic acids. The ability of sodium bicarbonate to convert the carboxylic acid into a soluble salt while leaving the phenol untouched is the key to this separation process.