Four pure liquids are summarized in the table below. All data are collected at 25 °C.
Liquid
Condensed structural formula
Molar mass (g mol⁻¹)
Capable of hydrogen bonding to ANOTHER molecule of the same liquid?
Permanent dipole moment (D)
A
CH₃CH₂CH₂CH₃
58
No
0.00
B
CH₃CH₂OH
46
Yes
1.69
C
CH₃OCH₃
46
No
1.30
D
HOCH₂CH₂OH
62
Yes
2.30
Two of these liquids are thoroughly mixed in a 1:1 molar ratio at 25 °C. After vigorous stirring, the mixture is allowed to stand. Two separate layers are observed at equilibrium.
Answer the following questions about the behavior of this system.
(a) Identify ONE specific pair of liquids from the table that is most likely to produce the two-layer system described. Justify your choice using intermolecular interactions for each liquid.
(b) For the pair you selected in part (a), predict whether the enthalpy of mixing, ΔHmix, is positive, negative, or approximately zero. Support your prediction with qualitative reasoning that references the relative strengths of the interactions broken and formed.
(c) Propose a molecular-level diagram (no drawing required—describe verbally) that depicts the spatial arrangement of the two liquids in the container after phase separation. Explain how the diagram you describe accounts for density differences and the observed layering.
(d) Suppose that small amounts of liquid C are now added to the container containing the separated layers from parts (a)–(c) and the system is again mixed. Predict and explain whether liquid C will preferentially dissolve in the upper layer, the lower layer, or distribute between both layers.