CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
Diethylene glycol (HOCH₂CH₂OCH₂CH₂OH, abbreviated DEG) is completely miscible with water at room temperature but separates from water at elevated temperatures. A student investigates this temperature-dependent miscibility and collects the following qualitative observations:
• At 20 °C: any volume ratio of DEG and water produces a single homogeneous liquid phase.
• At 70 °C: a 1:1 volume mixture separates into two distinct layers after standing.
• At 90 °C: the two layers produced at 70 °C merge into a single phase again.
The behavior is summarized as an "hourglass" or "closed-loop" miscibility gap in the temperature–composition diagram.
Respond to the prompts below.
(a) Describe, in molecular terms, why DEG and water exhibit complete miscibility at low temperature but phase-separate at an intermediate temperature. In your explanation, discuss both entropy and enthalpy considerations without performing numerical calculations.
(b) Provide a detailed rationale for why the two liquids become miscible again at a sufficiently high temperature, even though increased temperature generally disfavors hydrogen bonding.
(c) Sketch (verbally describe) the qualitative shape of the temperature versus DEG mole fraction phase diagram that is consistent with the observations. Label key regions and identify the critical solution temperatures involved.
(d) Suppose 5.0 mL of DEG is mixed with 5.0 mL of water at 70 °C and forms an upper DEG-rich layer and a lower water-rich layer. Predict which layer will have the higher refractive index and justify your answer based on molecular composition and polarizability.