Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Glacial acetic acid is obtained by (a) treating vinegar with dehydrating agents (b) crystallizing, separating and melting acetic acid (c) chemically separating acetic acid (d) distilling vinegar

Step-by-Step Solution

Answer

Option (b) is the correct choice: crystallizing, separating, and melting acetic acid gives glacial acetic acid.

Step 1: Understanding the Composition of Vinegar

Vinegar is typically composed of about 5-8% acetic acid in water. To obtain glacial acetic acid, the water content needs to be removed so that pure acetic acid is isolated.

Step 2: Exploring Glacial Acetic Acid Formation

Glacial acetic acid refers to the concentrated form of acetic acid, free from water. It is known as 'glacial' because it solidifies just below room temperature into a crystal-like solid that resembles ice.

Step 3: Analyzing the Options

- Option (a): Treating vinegar with dehydrating agents could remove water content, but might result in some impurities remaining. - Option (b): Crystallizing, separating and melting acetic acid would involve removing water physically by cooling it down, separating the acetic acid, and then melting it to obtain the pure substance once more. - Option (c): Chemically separating acetic acid doesn't specifically address water removal but suggests a separation process. - Option (d): Distilling vinegar would likely concentrate acetic acid but not in a pure, glacial form.

Step 4: Selecting the Most Appropriate Correct Answer

Considering the definition of glacial acetic acid, options should not just result in concentrated acetic acid but should also ensure purity. Option (b) accurately describes the process: the crystallization effectively separates the water from acetic acid at lower temperatures and subsequent melting results in pure acetic acid.

Key Concepts

Dehydration Process

The dehydration process is vital for obtaining glacial acetic acid, as it involves removing the water content present in vinegar. Vinegar consists of acetic acid and water, and to isolate the pure acetic acid, a dehydration step is necessary.

• Dehydrating agents can be used to chemically bind water molecules, converting them into forms that can be physically separated from the acetic acid.
• Common dehydrating agents include sulfuric acid and calcium oxide, though some might leave traces of impurities in the final product.

The main goal is to achieve a concentrated form of acetic acid, free of water and impurities. This requires precise control of the dehydration environment, as well as careful selection of agents and methods used.

Vinegar Composition

Understanding the composition of vinegar is key to extracting glacial acetic acid. Vinegar typically contains 5-8% acetic acid along with water and sometimes other components, depending on the source.

• Acetic acid is responsible for the sour taste in vinegar, and its concentration varies based on the vinegar type.
• Different types of vinegar may also contain trace amounts of flavoring agents or minerals.

The process of converting this acetic acid-rich solution into glacial acetic acid requires efficiently separating the acetic acid from its aqueous environment. This necessitates knowledge of both chemical properties and physical separation techniques.

Crystallization Method

Crystallization is a key method to produce pure acetic acid in its glacial form. It involves cooling the acetic acid solution until the acetic acid crystallizes out, leaving behind the water.

• As temperature drops, acetic acid begins to form crystals due to its lower solubility compared to water.
• This solid phase is essentially free of water, allowing it to be easily separated from the solution.

Once the crystals are formed, they are collected and melted back into their liquid form, ensuring a high purity level of glacial acetic acid. This method leverages the unique property of acetic acid solidifying just below room temperature, making it highly effective in obtaining the desired result.

Separation Techniques

Separating acetic acid from vinegar involves various techniques to ensure purity and high concentration, with crystallization being one of the most efficient. However, other methods such as distillation are also worth mentioning.

• Distillation involves heating the vinegar so that the components with lower boiling points, like water, evaporate first, leaving concentrated acetic acid.
• However, this process might not yield a fully pure substance, requiring additional steps for complete water removal.

Moreover, membrane-based techniques and extraction methods are alternative processes, although they are not as effective as crystallization in achieving glacial purity. Choosing the right technique depends on the desired purity and economic considerations.