Problem 157 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following mixture can be separated by steam distillation? (a) Benzoic acid and \(\mathrm{NaCl}\) (b) Acetone and methanol (c) Aniline and chlorobenzene (d) O- Nitrophenol and p-nitrophenol

Step-by-Step Solution

Answer

O-nitrophenol and p-nitrophenol can be separated by steam distillation.

Step 1: Understand Steam Distillation

Steam distillation is used to separate temperature-sensitive compounds. It works by boiling a mixture and passing steam through it, causing the volatile components to vaporize at a temperature lower than their boiling points and then condense back to liquid form. Components that are steam volatile can be separated using this method, particularly those immiscible with water.

Step 2: Analyze Each Option

Evaluate the options to identify mixtures where at least one component is steam volatile, and the components must have significantly different boiling points or be immiscible in each other.

Step 3: Option A Analysis

Benzoic acid and \(\mathrm{NaCl}\) do not present a case for steam distillation as \(\mathrm{NaCl}\) is not volatile and does not exhibit a boiling point where steam distillation would be applicable.

Step 4: Option B Analysis

Acetone and methanol are both polar solvents and are fully miscible with one another, meaning they do not separate easily by steam distillation as they require thermal separation based on different boiling points.

Step 5: Option C Analysis

Aniline and chlorobenzene both have close boiling points. Although chlorobenzene is slightly volatile, the closeness of their boiling points makes steam distillation impractical.

Step 6: Option D Analysis

O-nitrophenol and p-nitrophenol present a suitable case for steam distillation. O-nitrophenol is volatile in steam and can be separated from p-nitrophenol due to its lower boiling point.

Key Concepts

Volatile Compounds

Volatile compounds are substances that easily evaporate at low temperatures. This property makes them crucial in processes like steam distillation. They have high vapor pressures relative to their surroundings, allowing them to vaporize below their normal boiling points when heated. This is particularly useful in separating compounds from mixtures. For a compound to be considered volatile, it must be able to transform into vapor without requiring extreme heat.
Volatility depends on molecular characteristics, such as

• Intermolecular forces, where weaker forces make a compound more volatile.
• Molecular weight, often where lighter molecules are more volatile.

Understanding the volatility of compounds lets us predict their behavior in different temperature conditions. In the context of steam distillation, identifying which component of a mixture is volatile under steam helps determine if the method will be effective.

Boiling Points

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure, causing it to change from liquid to gas. In the realm of steam distillation, boiling points are essential because they allow certain volatile compounds to be distilled at temperatures lower than their usual boiling points.

Boiling points provide a key criterion for separating mixtures:

• Compounds with significantly different boiling points can be separated more easily.
• In steam distillation, the presence of steam lowers the effective boiling point of the more volatile component.

Thus, steam distillation is highly effective for compounds with varying boiling points, as it exploits the difference in temperatures at which components become gases. This approach is particularly useful for heat-sensitive compounds that might degrade at their normal boiling points.

Separation Techniques

Separation techniques are methods used to separate components of a mixture based on physical and chemical properties. Among these techniques, steam distillation plays a significant role. It exploits the immiscibility of certain compounds with water and their volatility in steam to achieve separation.

Key advantages of steam distillation include:

• Ability to distill at temperatures below the typical boiling point, preserving sensitive compounds.
• Effective for separating essential oils and aromatic compounds.

In practice, this method involves heating the mixture and introducing steam, causing the volatile component to vaporize and travel with the steam. This vapor cools and condenses separately from the non-volatile components, allowing for effective separation. Understanding these concepts allows one to predict the efficiency of steam distillation in different applications, ensuring optimal use in chemical processes.