# Problem: In the table we see that the viscosity of a series of hydrocarbons increases with molecular weight, doubling from the six-carbon molecule to the ten-carbon molecule.SubstanceFormulaViscosity (kg/m•s)HexaneCH3CH2CH2CH2CH2CH33.26 x 10–4HeptaneCH3CH2CH2CH2CH2CH2CH34.09 x 10–4OctaneCH3CH2CH2CH2CH2CH2CH2CH35.42 x 10–4NonaneCH3CH2CH2CH2CH2CH2CH2CH2CH37.11 x 10–4DecaneCH3CH2CH2CH2CH2CH2CH2CH2CH2CH31.42 x 10–3The surface tension of the hydrocarbon liquids in the table does increase from hexane to decane, but only by a rather small amount (20 % overall, compared to the doubling of viscosity). Which of the statements below is the most likely explanation for this phenomenon?(i) The flexibility of the molecules has a much larger effect on viscosity than on surface tension.(ii) Viscosity only depends on molecular weight, but surface tension depends on molecular weight and on intermolecular forces.(iii) Larger molecules can make larger liquid droplets and therefore have lower surface tension.

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###### Problem Details

In the table we see that the viscosity of a series of hydrocarbons increases with molecular weight, doubling from the six-carbon molecule to the ten-carbon molecule.

 Substance Formula Viscosity (kg/m•s) Hexane CH3CH2CH2CH2CH2CH3 3.26 x 10–4 Heptane CH3CH2CH2CH2CH2CH2CH3 4.09 x 10–4 Octane CH3CH2CH2CH2CH2CH2CH2CH3 5.42 x 10–4 Nonane CH3CH2CH2CH2CH2CH2CH2CH2CH3 7.11 x 10–4 Decane CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 1.42 x 10–3

The surface tension of the hydrocarbon liquids in the table does increase from hexane to decane, but only by a rather small amount (20 % overall, compared to the doubling of viscosity). Which of the statements below is the most likely explanation for this phenomenon?
(i) The flexibility of the molecules has a much larger effect on viscosity than on surface tension.
(ii) Viscosity only depends on molecular weight, but surface tension depends on molecular weight and on intermolecular forces.
(iii) Larger molecules can make larger liquid droplets and therefore have lower surface tension.