SOAPS & DETERGENTS — Process understanding. Question: During soap manufacture (saponification), how does adding common salt (NaCl) help in separating soap from the aqueous mixture?

Introduction / Context:
In the saponification process, triglycerides react with alkali to form soap (sodium or potassium salts of fatty acids) and glycerol. After the reaction, soap must be separated from the mother liquor. Traditional processes add common salt to “salt out” the soap. Understanding the physical chemistry behind this separation is key to mastering industrial soapmaking.

Given Data / Assumptions:

• Saponification yields a mixture of soap, glycerol, unreacted alkali, and water.
• Common salt (NaCl) is added to this mixture.
• We want the mechanism by which separation occurs.

Concept / Approach:
Adding electrolytes like NaCl reduces the solubility of soap in water (a “salting out” effect). The increased ionic strength favors the precipitation (or separation as curd) of the less soluble soap phase while glycerol and salt remain in the aqueous layer. This is not primarily a density manipulation; rather it is a solubility and activity effect driven by electrolyte interactions.

Step-by-Step Solution:
Recognize that soap is an amphiphilic salt; its solubility depends on ionic strength.Add NaCl → ionic strength increases.Higher ionic strength reduces soap solubility → soap separates as a distinct phase/precipitate.Glycerol remains in the brine, enabling decantation and recovery.

Verification / Alternative check:
Classical batch soapmaking demonstrates visible “curd” formation after salting, confirming the salting-out phenomenon common to many colloids and proteins as well.

Why Other Options Are Wrong:
A, C) Density change is not the operative separation mechanism.D) Increasing solubility would hinder, not aid, separation.E) Evaporation is not the purpose of adding salt.

Common Pitfalls:
Confusing salting out with flotation or with merely thickening the mixture. The key is reduced solubility due to electrolyte effects.

Final Answer:
By decreasing the solubility of the soap (salting out)