Lowering of Vapor Pressure on Mixing

When two liquids are mixed together, the vapor pressure of the resulting mixture can be different from the vapor pressures of the individual components. The vapor pressure of a liquid is the pressure exerted by its vapor when it is in equilibrium with its liquid phase at a given temperature. The vapor pressure depends on the intermolecular forces between the molecules of the liquid.

Aniline and Cyclohexane
When aniline and cyclohexane are mixed together, there is no significant difference in the intermolecular forces between the molecules of the two liquids. Aniline has hydrogen bonding while cyclohexane has only Van der Waals forces. The hydrogen bonding in aniline is stronger than the Van der Waals forces in cyclohexane. Therefore, the vapor pressure of the mixture is not significantly different from the vapor pressure of the individual components. Hence, there is no lowering of vapor pressure on mixing.

Chloroform and Acetone
When chloroform and acetone are mixed together, the intermolecular forces between the molecules of the two liquids are different. Chloroform has dipole-dipole interactions, while acetone has both dipole-dipole interactions and hydrogen bonding. The hydrogen bonding in acetone is stronger than the dipole-dipole interactions in chloroform. When the two liquids are mixed, the stronger hydrogen bonding in acetone disrupts the intermolecular forces in chloroform, leading to a lowering of the vapor pressure of the mixture. Hence, there is a lowering of vapor pressure on mixing chloroform and acetone.

Ethyl Alcohol and Pentane
When ethyl alcohol and pentane are mixed together, there is a significant difference in the intermolecular forces between the molecules of the two liquids. Ethyl alcohol has hydrogen bonding while pentane has only Van der Waals forces. The hydrogen bonding in ethyl alcohol is stronger than the Van der Waals forces in pentane. When the two liquids are mixed, the stronger hydrogen bonding in ethyl alcohol disrupts the intermolecular forces in pentane, leading to a lowering of the vapor pressure of the mixture. Hence, there is a lowering of vapor pressure on mixing ethyl alcohol and pentane.

Dimethyl Ether and Chloroform
When dimethyl ether and chloroform are mixed together, there is no significant difference in the intermolecular forces between the molecules of the two liquids. Both dimethyl ether and chloroform have dipole-dipole interactions. The dipole-dipole interactions in both liquids are comparable. Therefore, the vapor pressure of the mixture is not significantly different from the vapor pressure of the individual components. Hence, there is no lowering of vapor pressure on mixing dimethyl ether and chloroform.

In conclusion, the solutions that show a lowering of vapor pressure on mixing are chloroform and acetone (option B) and dimethyl ether and chloroform (option D).