Question

A mixture of benzoic acid and naphthalene can be separated by crystallization from: (a) Cold water (b) Ether (c) Hot water (d) Benzene

Short answer

The mixture can be separated by crystallization using hot water.

Step-by-step solution

Understand Solubility

First, we need to understand the solubility behavior of benzoic acid and naphthalene in different solvents. Benzoic acid is soluble in hot water but not in cold water, and naphthalene is generally insoluble in water. Both substances are soluble in organic solvents like ether and benzene.

Analyze Solvent Options for Crystallization

Let's analyze the options: (a) Cold water: Benzoic acid may not dissolve well, naphthalene will not dissolve. (b) Ether: Both benzoic acid and naphthalene are soluble. (c) Hot water: Benzoic acid dissolves, naphthalene does not dissolve. (d) Benzene: Both benzoic acid and naphthalene dissolve.

Select the Appropriate Solvent

For crystallization, we need a solvent in which one component dissolves while the other remains insoluble at a particular temperature. Hot water is a good choice because benzoic acid dissolves in hot water and can crystallize out upon cooling, leaving naphthalene behind.

Conclusion

After assessing the solubility data, the best solvent for the crystallization separation of benzoic acid and naphthalene is hot water, as it allows for differential solubility based on temperature.

Key concepts

Solubility

The concept of solubility revolves around the ability of a substance to dissolve in a solvent. This property is crucial because it determines how we can separate different substances in a mixture. For example, when we talk about solubility in the context of benzoic acid and naphthalene, we consider how well these compounds dissolve in various solvents.
Solubility can vary based on temperature, the nature of the solute, and the solvent in use. Generally, an increase in temperature increases solubility for solids in liquids, but this can vary depending on the substance. That's why understanding specific solubility rules for each compound is essential to make the right decision when it comes to separating them.

Benzoic Acid

Benzoic acid is a white, crystalline solid that is slightly soluble in cold water but becomes more soluble in hot water. This property makes it unique when separating it from other compounds using the crystallization method. Its solubility in hot water can be utilized to separate it from substances that do not dissolve under such conditions.
The process works by dissolving benzoic acid in hot water, then allowing the solution to cool. Upon cooling, the benzoic acid will crystallize out of the solution, enabling easy separation from insoluble components like naphthalene. This efficient usage of temperature differences plays a key role in successfully applying solvent crystallization for separating mixtures.

Naphthalene

Naphthalene is a polycyclic aromatic hydrocarbon known for its lack of solubility in water. It is notable for being used in mothballs due to its strong odor and sublimation properties. In terms of crystallization, its insolubility in both cold and hot water is an advantage when aiming to separate it from other compounds.
Because naphthalene does not dissolve well in water, it remains in its solid form even when benzoic acid is dissolved. Therefore, when employing a solvent like hot water that only dissolves the benzoic acid, naphthalene remains as a solid residue, proving to be a practical choice for methods involving differential solubility.

Differential Solubility

Differential solubility is a strategic approach used in chemistry to separate components of a mixture based on their differing solubility in a chosen solvent. By carefully selecting a solvent where one component is highly soluble and another is not, chemists can achieve effective separation through crystallization.
In the case of benzoic acid and naphthalene, differential solubility comes into play with hot water. The key is that benzoic acid dissolves in hot water while naphthalene does not. Upon cooling, the benzoic acid crystallizes out, leaving the less soluble naphthalene behind. This method is incredibly useful for separating compounds based on temperature-dependent solubility differences. This is why understanding the solubility characteristics of each component and their interaction with various solvents is vital in chemical separations.