Question

In liquid-liquid extraction, what is the primary purpose of using a solvent?

• A. To increase the solubility of solutes
• B. To separate desired components from a mixture based on differences in solubility
• C. To facilitate phase separation of the components
• D. To speed up the chemical reaction between components

Hint:

Every separation process has a specific driving force:
Distillation \(\implies\) Difference in vapor pressures (volatilities).
Gas Absorption \(\implies\) Solubility of a gas in a liquid.
Liquid-Liquid Extraction \(\implies\) Difference in solubilities of a solute in two immiscible liquids.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary thermodynamic and operational purpose of introducing an external liquid solvent in a liquid-liquid extraction (solvent extraction) process.

Step 2: Key Formula or Approach:
Liquid-liquid extraction is a mass transfer separation technique used to separate the components of a liquid solution (feed) by contacting it with another immiscible or partially miscible liquid phase (the solvent).
The separation relies on the distribution of a solute between the two liquid phases, which is quantified by the distribution coefficient (or partition coefficient) \(K_D\):
\[ K_D = \frac{x_E}{x_R} \] where \(x_E\) and \(x_R\) are the equilibrium concentrations of the solute in the extract (solvent-rich) phase and the raffinate (carrier-rich) phase, respectively.

Step 3: Detailed Explanation:
Let us evaluate the purposes described in the options:
- Option (A) states that the purpose is to increase solubility of solutes. While the solute must be highly soluble in the solvent, the overall goal of the operation is separation, not simply increasing solubility.
- Option (B) states that the solvent is used to separate desired components from a mixture based on differences in solubility. This is the exact fundamental principle of extraction. The solute has a higher affinity (solubility) for the added solvent than for the original carrier liquid in the feed, enabling it to transfer from the feed phase to the solvent phase.
- Option (C) mentions facilitating phase separation. Phase separation (density-driven settling) is a physical requirement to recover the liquids, but it is a consequence of the immiscibility of the liquids, not the primary driving force for solute mass transfer.
- Option (D) mentions speeding up a chemical reaction. This is incorrect because liquid-liquid extraction is a purely physical separation process, and no chemical reaction is intended between the components.
Thus, the primary purpose is the separation based on solubility differences.

Step 4: Final Answer
Therefore, the correct option is (B).