Which of the following solutions is not nearly ideal in behaviour?

Question

Accepted Answer

Option 4, which is the combination of carbon disulphide (CS2) and acetone, is not nearly ideal in behavior. Here's why:

1. **Explanation for Option 4 (Correct Answer):**
   - **Non-Ideal Behavior:** Carbon disulphide and acetone form a solution that exhibits significant deviations from ideal behavior. This is primarily due to the strong dipole-dipole interactions between acetone molecules and the relatively weak interactions between carbon disulphide and acetone. Acetone is a polar molecule with a significant dipole moment, while carbon disulphide is non-polar. When mixed, the disparity in intermolecular forces leads to non-ideal behavior, often resulting in positive deviations from Raoult's Law. This means the vapor pressure of the solution is higher than expected for an ideal solution, indicating weaker interactions between the dissimilar molecules compared to the pure components.

2. **Explanation for Other Options (Incorrect Answers):**
   - **Option 1: n-Hexane and n-Heptane**
     - **Nearly Ideal Behavior:** n-Hexane and n-heptane are both non-polar hydrocarbons with similar molecular structures and sizes. The intermolecular forces (primarily London dispersion forces) between them are similar to those in the pure components, leading to nearly ideal behavior. Their similar nature results in minimal deviation from Raoult's Law.

- **Option 2: Bromoethane and Chloroethane**
     - **Nearly Ideal Behavior:** Bromoethane and chloroethane are both haloalkanes with similar molecular sizes and polarities. The interactions between these molecules are similar to those within the pure substances, resulting in nearly ideal behavior. The slight difference in polarity does not significantly affect the overall interactions, allowing the solution to behave almost ideally.

- **Option 3: Benzene and Toluene**
     - **Nearly Ideal Behavior:** Benzene and toluene are both aromatic hydrocarbons with similar structures. The interactions between benzene and toluene molecules are very similar to those within the pure liquids, primarily involving π-π interactions. This similarity leads to nearly ideal behavior, with minimal deviations from Raoult's Law.

In summary, Option 4 is the correct answer because the combination of carbon disulphide and acetone results in a solution that exhibits significant non-ideal behavior due to the disparity in intermolecular forces between the two components. The other options involve pairs of substances with similar molecular characteristics, leading to nearly ideal behavior.