When NaCl is added to the reaction mixture of an oil and caustic soda, the soap is thrown out because

Options

A. NaCl is an ionic compound
B. soap is insoluble in the presence of chloride ions
C. the solubility product of NaCl decreases in the presence of soap
D. the solubility product of the soap is exceeded due to the increased concentration of `Na^+` ions

Answer

To solve the question of why soap is thrown out when NaCl is added to the reaction mixture of oil and caustic soda, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Soap Formation**:
- Soap is formed through a process called saponification, which involves the reaction of a fatty acid (from oil) with an alkali (like caustic soda, NaOH). This reaction produces soap (sodium salt of fatty acid) and glycerol.

2. **Composition of Soap**:
- The soap can be represented as RCOO⁻Na⁺, where R is the long hydrocarbon chain derived from the fatty acid. The soap consists of negatively charged RCOO⁻ ions and positively charged Na⁺ ions.

3. **Adding NaCl**:
- When NaCl (sodium chloride) is added to the soap solution, it dissociates into Na⁺ and Cl⁻ ions in the solution. This increases the concentration of Na⁺ ions in the mixture.

4. **Effect on Solubility Product (Ksp)**:
- The solubility product (Ksp) of soap is a constant that depends on the concentrations of its ions (RCOO⁻ and Na⁺). When the concentration of Na⁺ ions increases due to the addition of NaCl, the product of the concentrations of the ions (RCOO⁻ and Na⁺) can exceed the Ksp value.

5. **Precipitation of Soap**:
- When the product of the concentrations of the ions exceeds the Ksp, the soap becomes less soluble and precipitates out of the solution. This phenomenon is known as "salting out."

6. **Conclusion**:
- Therefore, the addition of NaCl leads to the precipitation of soap from the solution because the increased concentration of Na⁺ ions causes the solubility product to be exceeded.

### Final Answer:
When NaCl is added to the reaction mixture of oil and caustic soda, the soap is thrown out due to the increased concentration of Na⁺ ions, which causes the solubility product (Ksp) of the soap to be exceeded, resulting in its precipitation. This process is known as salting out.

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