9. Which substance is expected to exhibit the greatest surface tension at 25°C?
a. CH3OCH3
b. C2H5OH
c. CH3CH(OH)CH3
d. CH2(OH)CH2OH
CH2(OH)CH2OH exhibits the greatest surface tension at 25°C among the given options.
Surface tension arises from cohesive intermolecular forces, primarily hydrogen bonding in these polar molecules. Ethylene glycol has two -OH groups, enabling extensive hydrogen bonding networks that maximize surface tension.
Option Analysis
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CH3OCH3 (Dimethyl ether): Lacks hydrogen bonding capability; relies on weak dipole-dipole forces. Surface tension measures around 12.6 mN/m near room temperature, the lowest here.
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C2H5OH (Ethanol): Forms hydrogen bonds via one -OH group. Surface tension is 22.32 dyn/cm (22.32 mN/m) at 20°C, slightly higher at 25°C.
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CH3CH(OH)CH3 (Isopropanol): One -OH group but bulkier structure reduces hydrogen bonding efficiency. Surface tension is about 20.93 mN/m at 25°C.
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CH2(OH)CH2OH (Ethylene glycol): Two -OH groups allow multiple hydrogen bonds per molecule. Surface tension reaches 47.99 dyn/cm (47.99 mN/m) at 25°C, highest due to strongest cohesion.
Introduction to Surface Tension
Surface tension determines how liquids minimize surface area due to intermolecular forces. In greatest surface tension at 25°C comparisons, hydrogen bonding strength dictates the winner among CH3OCH3 (dimethyl ether), C2H5OH (ethanol), CH3CH(OH)CH3 (isopropanol), and CH2(OH)CH2OH (ethylene glycol). This MCQ tests understanding of cohesive forces for exams like CSIR NET Life Sciences.
Hydrogen Bonding Impact
Hydrogen bonding occurs when H attaches to O, N, or F, creating strong attractions.
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Ethers like CH3OCH3 show dipole-dipole only (~12-13 mN/m).
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Monohydric alcohols (C2H5OH ~22 mN/m, CH3CH(OH)CH3 ~21 mN/m) form fewer bonds.
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Dihydric CH2(OH)CH2OH forms extensive networks (~48 mN/m).
| Substance | Formula | -OH Groups | Surface Tension (mN/m at ~25°C) | Intermolecular Force |
|---|---|---|---|---|
| Dimethyl ether | CH3OCH3 | 0 | 12.6 | Dipole-dipole |
| Ethanol | C2H5OH | 1 | 22.3 | H-bonding |
| Isopropanol | CH3CH(OH)CH3 | 1 | 20.9 | H-bonding |
| Ethylene glycol | CH2(OH)CH2OH | 2 | 48.0 | Strong H-bonding |
Exam Solution
Answer: d. CH2(OH)CH2OH. More -OH groups mean greater cohesion, confirmed by data. Ethanol > isopropanol due to less steric hindrance. Ideal for CSIR NET practice on physical properties of biomolecules.
