A laboratory stores an unlabeled bottle containing a clear colorless liquid known to be one of the following three molecular compounds: methanol (CH3OH), ethylene glycol (HOCH2CH2OH), or n-butane (C4H10). To identify the liquid, a student performs three independent measurements at 20 °C:
• Dynamic viscosity: 1.9 mPa·s
• Surface tension: 24.7 mN·m⁻¹
• Density: 0.791 g·cm⁻³
Additional reference data at 20 °C are provided:
Compound
Viscosity (mPa·s)
Surface tension (mN·m⁻¹)
Density (g·cm⁻³)
Methanol
0.59
22.1
0.791
Ethylene glycol
16.7
48.9
1.113
n-Butane (liquid)
0.24
12.6
0.584
Answer the following questions to determine the identity of the unknown liquid.
(a) Compare the student’s measured viscosity, surface tension, and density values with the reference data. For each property, calculate the percent difference between the measured value and the closest matching reference value.
(b) Using the results from part (a), determine which of the three candidate compounds most likely matches the unknown. Justify your answer quantitatively.
(c) Provide a detailed molecular-level explanation that connects the dominant intermolecular forces in the identified compound to the relative magnitudes of viscosity and surface tension for all three compounds.
(d) Suppose the laboratory temperature inadvertently rises to 50 °C. Predict qualitatively how each of the three measured properties for the identified compound will change. Justify each prediction in terms of molecular motion and intermolecular forces.